Correlator and receiving apparatus utilizing the same

ABSTRACT

I delay units and Q delay units continuously delay values of in-phase component and quadrature component of an inputted first baseband received signal. I storage units and Q storage units store each component of a waveform of STS (Short Training Sequence) corresponding to a first transmitting antenna. Multipliers carry out multiplication in correlation processing. An adder adds up multiplication results outputted from the multipliers and then another adder adds up the addition result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to correlation technologies, and it particularly relates to a correlator for computing the correlation among received signals and a receiving apparatus using the correlator.

2. Description of the Related Art

In wireless communication, it is generally desired that the limited frequency resources be used effectively. Adaptive array antenna technology is one of the technologies that realize the effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. In other words, the apparatuses provided with adaptive array antennas change respectively the amplitudes and phases of signals received by a plurality of antennas, add up the thus changed received signals, and receive signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in said amplitude and phase (hereinafter referred to as “weight”). And the signals are transmitted in a directional pattern of the antenna corresponding to the weight.

One example of processings to compute the weights in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula whose amount of calculation is smaller than that required to directly solve the Wiener solution. For such recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used. On the other hand, for the purpose of realizing a higher data transmission rate and improving the transmission quality, there is a case where data are subjected to multi-carrier modulation and the multi-carrier signals are transmitted (See reference (1) in the following Related Art List, for instance).

Related Art List

-   (1) Japanese Patent Application Laid-Open No. Hei10-210099.

There is a MIMO (Multiple Input Multiple Output) system as one of technologies by which to raise the transmission rate of data using the adaptive array antenna technology. A transmitting apparatus and a receiving apparatus in the MIMO system are each equipped with a plurality of antennas, and a channel suitable for each of the antennas is set. That is, the channel of up to the maximum number of antennas is set for the communication between the transmitting apparatus and the receiving apparatus, so as to improve the data transmission rate. Moreover, combining the MIMO system with a technique of transmitting multi-carrier signals results in a higher data transmission rate. On the other hand, the transmitted signal generally contains a preamble that is a known signal in order for the signal transmitted from the transmitting apparatus to be accurately received, and a plurality of preambles are generally contained in a signal if a MIMO system is employed in particular. When a receiving apparatus in a MIMO system receives a plurality of preambles and the timing is detected by performing a correlation processing on the plurality of received preambles, a plurality of correlators need to be provided. Thus, the processing amount thereof and the scale of a receiving apparatus will be increased.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing circumstances and an object thereof is to provide a correlator which simplifies processings even when a correlation processing is carried out to multi-carrier known signals transmitted from a plurality of antennas and also to provide a receiving apparatus utilizing said correlator.

A preferred embodiment according to the present invention relates to a correlator. This correlator comprises: an input unit which sequentially inputs signals in which a plurality of series are combined wherein the plurality of series are defined beforehand in a form such that a plurality of reference signals among the series are respectively changed for the series in which the plurality of reference signals are arranged on time axis; a delay unit which continuously delays the inputted signals; a plurality of storage units which store respectively a plurality of reference signals corresponding one of the plurality of series; and a correlation processing unit which performs correlation processing based on values of the plurality of delayed signals and values of the plurality of stored reference signals and which outputs respectively a plurality of values of correlation between the inputted signals and the plurality of series. In this correlator, a plurality of series contained in the signals inputted to the input unit may have a predetermined relationship, and the correlation processing unit may perform the correlation processing by combination of multiplication and addition according to the predetermined relationship.

By employing the above correlator, the multiplication and the addition are combined in accordance with the predetermined relationship, so that a processing amount can be reduced.

The signals inputted to the input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have may be such that a value of in-phase component of a reference signal contained in one of the plurality of series is equal to a value of quadrature component of a reference signal contained in the other thereof whereas a value of quadarature component of the reference signal contained in one of the plurality of series is equal to a value of in-phase component of the reference signal contained in the other thereof, and the correlation processing unit may put to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sum up in different combinations a plurality of multiplication results generated by the multiplication.

If a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then the correlation processing unit may perform the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and a difference between the second value and the first value are calculated for the other of the two series.

Signals to be inputted to the input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there may be a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas; the storage unit may store, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals; and the correlator may further comprise a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation,

The signals inputted to the input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have may be such that an absolute value of in-phase component of a reference signal contained in one of the plurality of series is equal to an absolute value of quadrature component of a reference signal contained in the other thereof and the sign thereof is inverted whereas an absolute value of quadarature component of the reference signal contained in one of the plurality of series is equal to an absolute value of in-phase component of the reference signal contained in the other thereof and the sign thereof is inverted, and the correlation processing unit may put to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sum up in different combinations a plurality of multiplication results generated by the multiplication.

If a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then the correlation processing unit may perform the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and then inverting the sign of resulting summation and a difference between the second value and the first value and then inverting the sign of resulting difference are calculated for the other of the two series.

By employing the above correlator, the multiplication among a plurality of correlation values can be shared when multiplying the plurality of correlation values, so that the processing amount thereof can be made small.

Signals to be inputted to the input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there may be a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to an absolute value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas and the sign thereof is inverted; the storage unit may store, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals; and the correlator may further comprise a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation,

The plurality of series contained in the signals inputted to the input unit may be defined in a manner such that each value thereof varies in a predetermined cycle, and the predetermined relationship may be defined in a manner such that if one of the plurality of series is assigned as a reference, then a cycle of the series assigned as the reference is an integral multiple of a cycle of series other than the series assigned as the reference; the plurality of storage units may store respectively a plurality of reference signals corresponding to the series assigned as the reference; and the correlation processing unit may select part of the plurality of stored reference signals for the series other than the series assigned as the reference, based on a difference between the cycle of series assigned as the reference and the cycle of series other than the series assigned as the reference, and carry out multiplication between the thus partially selected reference signal and the plurality of delayed input signals.

By employing the above correlator, the storage unit for computing a plurality of correlation values can be shared, so that the processing amount can be made small.

The input signals inputted to the input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have may be such that only one of in-phase component and quadrature component of the reference signal has a predetermined value; the storage unit may respectively store information by which to invert the sign of the delayed signals in such a form as to correspond to the plurality of reference signals; and as the correlation processing the correlation processing unit may add up a signal which has been inverted based on the information and the delayed signal.

By employing the above correlator, no multiplication is necessary when computing a plurality of correlation values, so that the processing amount can be made small.

Another preferred embodiment according to the present invention relates to a receiving apparatus. This receiving apparatus comprises: an input unit which sequentially inputs signals in which a plurality of series are combined wherein the plurality of series are defined beforehand in a form such that a plurality of reference signals among the series are respectively changed for the series in which the plurality of reference signals are arranged on time axis; a delay unit which continuously delays the inputted signals; a plurality of storage units which store respectively a plurality of reference signals corresponding one of the plurality of series; a correlation processing unit which performs correlation processing based on values of the plurality of delayed signals and values of the plurality of stored reference signals and which outputs respectively a plurality of values of correlation between the inputted signals and the plurality of series; and a control unit which detects timing of the inputted signals based on the plurality of values of correlation. In this receiving apparatus, a plurality of series contained in the signals inputted to the input unit may have a predetermined relationship, and the correlation processing unit may perform the correlation processing by combination of multiplication and addition according to the predetermined relationship.

The signals inputted to the input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have may be such that a value of in-phase component of a reference signal contained in one of the plurality of series is equal to a value of quadrature component of a reference signal contained in the other thereof whereas a value of quadarature component of the reference signal contained in one of the plurality of series is equal to a value of in-phase component of the reference signal contained in the other thereof; and the correlation processing unit in the receiving apparatus may put to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sum up in different combinations a plurality of multiplication results generated by the multiplication.

If a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then the correlation processing unit in the receiving apparatus may perform the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and a difference between the second value and the first value are calculated for the other of the two series.

Signals to be inputted to the input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there may be a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas; the storage unit in the receiving apparatus may store, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals; and the receiving apparatus may further comprise a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation.

The signals inputted to the input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have may be such that an absolute value of in-phase component of a reference signal contained in one of the plurality of series is equal to an absolute value of quadrature component of a reference signal contained in the other thereof and the sign thereof is inverted whereas an absolute value of quadarature component of the reference signal contained in one of the plurality of series is equal to an absolute value of in-phase component of the reference signal contained in the other thereof and the sign thereof is inverted; and the correlation processing unit in the receiving apparatus may put to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sum up in different combinations a plurality of multiplication results generated by the multiplication.

If a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then the correlation processing unit in the receiving apparatus may perform the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and then inverting the sign of resulting summation and a difference between the second value and the first value and then inverting the sign of resulting difference are calculated for the other of the two series.

Signals to be inputted to the input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there may be a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to an absolute value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas and the sign thereof is inverted; the storage unit may store, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals; and the receiving apparatus may further comprise a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation.

It is to be noted that any arbitrary combination of the above-described structural components and expressions changed among a method, an apparatus, a system, a recording medium, a computer program and so forth are all effective as and encompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a spectrum of a multi-carrier signal according to a first embodiment of the present invention.

FIG. 2 illustrates a concept of a communication system according to a first embodiment of the present invention.

FIG. 3 shows a structure of a burst format according to a first embodiment of the present invention.

FIG. 4 illustrates a structure of the transmitting apparatus shown in FIG. 2.

FIG. 5 illustrates a structure of a receiving apparatus 12 shown in FIG. 2.

FIG. 6 illustrates a structure of the first radio unit shown in FIG. 5.

FIG. 7 illustrates a structure of the first processing unit shown in FIG. 5.

FIGS. 8A to 8C show structures of burst formats according to a first embodiment of the present invention.

FIGS. 9A and 9B illustrate waveforms of known signals transmitted from the transmitting apparatus shown in FIG. 4.

FIGS. 10A to 10C illustrate waveforms of known signals transmitted from the transmitting apparatus shown in FIG. 2.

FIG. 11 is a flowchart showing a procedure of a receiving operation by the receiving apparatus of FIG. 5.

FIGS. 12A and 12B illustrate waveforms of known signals transmitted from a transmitting apparatus according to a third embodiment of the present invention.

FIG. 13 illustrates a structure of a correlator according to a third embodiment of the present invention.

FIGS. 14A and 14B illustrate waveforms of known signals transmitted from a transmitting apparatus according to a third embodiment of the present invention.

FIG. 15 illustrates a structure of a correlator according to a fourth embodiment of the present invention.

FIG. 16 illustrates a waveform of a known signal transmitted from a transmitting apparatus according to a fifth embodiment of the present invention.

FIG. 17 illustrates a structure of a correlator according to a fifth embodiment of the present invention.

FIG. 18 shows a relationship between the number of transmitting antennas for transmitting data and the patterns of STSs transmitted from transmitting antennas, according to a sixth embodiment of the present invention.

FIG. 19 illustrates a waveform of STSa shown in FIG. 18.

FIG. 20 illustrates a waveform of STSb shown in FIG. 18.

FIG. 21 illustrates a waveform of STS1 shown in FIG. 18.

FIG. 22 illustrates a waveform of STS2 shown in FIG. 18.

FIG. 23 is a flowchart showing a procedure of a receiving operation by a receiving apparatus according to a sixth embodiment of the present invention.

FIG. 24 shows a relationship between the number of transmitting antennas for transmitting data and the patterns of STSs transmitted from the transmitting antennas, according to a seventh embodiment of the present invention.

FIG. 25 illustrates a waveform of STSb′ shown in FIG. 24.

FIG. 26 illustrates a structure of a correlator according to a seventh embodiment of the present invention.

FIG. 27 illustrates a structure of a correlator according to an eighth embodiment of the present invention.

FIGS. 28A to 28D illustrate, in outline, known signals placed at subcarriers according to a ninth embodiment of the present invention.

FIG. 29 illustrates a structure of a transmitting apparatus according to a tenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on the following embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiments are not necessarily essential to the invention.

First Embodiment

Before describing the present invention in detail, an outline of the present invention will be described first. A first embodiment of the present invention relates to a MIMO system which comprises a transmitting apparatus equipped with a plurality of antennas and a receiving apparatus equipped with a plurality of antennas. The MIMO system according to the present embodiment transmits signals by multi-carriers, or more specifically OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, and the transmitted signals form burst signals. A preamble signal is placed at the head of a burst signal. And a receiving apparatus, which has received a signal, carries out the setting of AGC, the synchronization of timing, the recovery of carriers and the like, based on the preamble signal. In a MIMO system, independent signals are transmitted from a plurality of antennas of a transmitting apparatus and a receiving apparatus demodulates desired signals by separating received signals by adaptive array signal processing. During the period of a preamble signal, however, the weighting for an adaptive array signal processing is not complete, so that the signals cannot be fully separated by an adaptive array signal processing. A transmitting apparatus according to the present embodiment defines a plurality of preamble signals in such a way as to cause a smaller correlation between a plurality of preamble signals transmitted respectively by a plurality of antennas. As a result, even when the separation of signals by an adaptive array signal processing is not adequate, there is little interference among the preamble signals.

FIG. 1 illustrates a spectrum of a multi-carrier signal according to the first embodiment. FIG. 1 shows a spectrum of a signal in a wireless LAN (Local Area Network) conforming to the IEEE802.11a standard as a wireless system employing an OFDM modulation scheme. One of a plurality of carriers in an OFDM scheme is generally called a subcarrier. Herein, however, each of the subcarriers is designated by a “subcarrier number”. As illustrated in FIG. 1, the IEEE802.11a standard defines 53 subcarriers, namely, subcarrier numbers “−26” to “26”. It is to be noted that the subcarrier number “0” is set to null so as to reduce the effect of a direct current component in a baseband signal. Also, the respective subcarriers are modulated by BPSK, QSPK, 16 QAM and 64 QAM.

FIG. 2 illustrates a concept of a communication system 100 according to the first embodiment. The communication system 100 includes a transmitting apparatus 10 and a receiving apparatus 12. The transmitting apparatus 10 includes a first transmitting antenna 14 a and a second transmitting antenna 14 b, which are generically called transmitting antennas 14, and the receiving apparatus 12 includes a first receiving antenna 16 a and a second receiving antenna 16 b, which are generically called receiving antennas 16.

The transmitting apparatus 10 transmits signals, whereas the first transmitting antenna 14 a and the second transmitting antenna 14 b transmit different signals. The receiving apparatus 12 receives the signals transmitted from the first transmitting antenna 14 a and the second transmitting antenna 14 b by the first receiving antenna 16 a and the second receiving antenna 16 b. The receiving apparatus 12 separates received signals by adaptive array signal processing and demodulates the signals transmitted from the first transmitting antenna 14 a and the second transmitting antenna 14 b independently. Here, if channel characteristic between the first transmitting antenna 14 a and the first receiving antenna 16 a is denoted by h₁₁, that between the first transmitting antenna 14 a and the second receiving antenna 16 b by h₁₂, that between the second transmitting antenna 14 b and the first receiving antenna 16 a by h₂₁, and that between the second transmitting antenna 14 b and the second receiving antenna 16 b by h₂₂, then the receiving apparatus 12 operates in such a manner as to activate h₁₁ and h₂₂ only by an adaptive array signal processing and demodulate the signals transmitted from the first transmitting antenna 14 a and the second transmitting antenna 14 b independently.

FIG. 3 shows a structure of a burst format according to the first embodiment. However, this does not correspond to a MIMI system. Instead, this burst format corresponds to a traffic channel of IEE802.11a standard. In the OFDM modulation scheme, the total sum of the size of Fourier transform and the number of symbols of a guard interval generally constitutes one unit. This “one unit” is called an OFDM symbol in the present embodiment. In the IEEE802.11 standard, the size of Fourier transform is 64 (hereinafter the points of one FFT (Fast Fourier Transform) will be called “FFT point”) and the FFT point number of a guard interval is 16, so that the OFDM symbol corresponds to 80 FFT points.

As illustrated in FIG. 3, a “preamble” composed of “4 OFDM symbols” is placed in the leading portion of a burst signal. And a “signal” of “1 OFDM symbol” and “data” of arbitrary length in this order are placed subsequent to the “preamble”. The “preamble” is a known signal used for the setting of AGC, timing synchronization and carrier recovery and the like in the receiving apparatus 12. The “signal” is a control signal whereas the “data” is information to be transmitted from the transmitting apparatus 10 to the receiving apparatus 12. As shown also in FIG. 3, the “preamble” composed of “4 OFDM symbols” are separated into “STS (Short Training Sequence)” composed of “2 OFDM symbols” and “LTS (Long Training Sequence)” composed of “2 OFDM symbols”. STS is constituted by ten signal units “t₁” to “t₁₀” and each signal unit, such as “t₁”, is equal to 16 FFT points. Though STS is 16 FFT points in time-domain unit as described above, it uses, in frequency domain, 12 subcarriers among 53 subcarriers as shown in FIG. 1. STS is used particularly for the setting of AGC and the timing synchronization. LTS, on the other hand, is constituted by two signal units “T₁” and “T₂” and a guard interval “GI2” which is twice as long as “t₁”. And one signal unit such as “T₁” is 64 FFT points whereas “GI2” is 32 FFT points. LTS is used particularly for the carrier recovery.

A signal in the frequency domain as shown in FIG. 1 is expressed by S_(−26, 26), where the subscript indicates the subcarrier number. Using this notation, STS of IEEE802.11a standard is expressed as in the following equation (1). S _(−26,26) =sqrt(13/6){0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0−1−j,0,0,0,1+j,0,0,0,0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0}  (1) where “1+j” denotes the signal point of STS after a QPSK modulation.

Now problems to be solved when STS of IEEE802.11a standard is transmitted from each of the first transmitting antenna 14 a and the second transmitting antenna 14 b shown in FIG. 2 will be explained. If the signal transmitted from the first transmitting antenna 14 a is S₁(t), the signal transmitted from the second transmitting antenna 14 b is S₂(t), and the noise contained in the signals is n_(1(t) and n) ₂(t), then X₁(t), or the signal received by the first receiving antenna 16 a, and X₂(t), or the signal received by the second receiving antenna 16 b, will be expressed as: X ₁(t)=h ₁₁ S ₁(t)+h ₂₁ S ₂(t)+n ₁(t)X ₂(t)=h ₁₂ S ₁(t)+h ₂₂ S ₂(t)+n ₂(t)   (2) The signal strength in 16 FFT of signals received by the first receiving antenna 16 a is expressed as follows: $\begin{matrix} \begin{matrix} {{\sum{{X_{1}(t)}}^{2}} = {\sum{{X_{1}(t)}{X_{1}^{*}(t)}}}} \\ {= {\sum\left\{ {{h_{11}{S_{1}(t)}} + {h_{21}{S_{2}(t)}} + {n_{1}(t)}} \right\}}} \\ {\left\{ {{h_{11}^{*}{S_{1}^{*}(t)}} + {h_{21}^{*}{S_{2}^{*}(t)}} + {n_{1}^{*}(t)}} \right\}} \\ {= {{h_{11}h_{11}^{*}{\sum{{S_{1}(t)}{S_{1}^{*}(t)}}}} + {h_{21}h_{21}^{*}{\sum{{S_{2}(t)}{S_{2}^{*}(t)}}}} +}} \\ {{h_{11}h_{21}^{*}{\sum{{S_{1}(t)}{S_{2}^{*}(t)}}}} + {h_{11}^{*}h_{21}{\sum{{S_{1}^{*}(t)}{S_{2}(t)}}}} +} \\ {{h_{11}{\sum{{S_{1}(t)}{n_{1}^{*}(t)}}}} + {h_{21}{\sum{{S_{2}(t)}{n_{1}^{*}(t)}}}} +} \\ {{h_{11}^{*}{\sum{{S_{1}^{*}(t)}{n_{1}(t)}}}} + {h_{21}^{*}{\sum{{S_{2}^{*}(t)}{n_{1}(t)}}}} +} \\ {\sum{{n_{1}(t)}{n_{1}^{*}(t)}}} \end{matrix} & (3) \end{matrix}$

Using the relations ΣS₁*(t)S₂(t)=Xc, ΣS_(i)*(t)n_(j)(t)=0 and |n_(j)(t)|²≈0, the strength is expressed by: $\begin{matrix} \begin{matrix} {{\sum{{X_{1}(t)}}^{2}} = {{h_{11}}^{2} + {h_{21}}^{2} + {h_{11}h_{21}^{*}{Xc}^{*}} + {h_{11}^{*}h_{21}{Xc}}}} \\ {= {{h_{11}}^{2} + {h_{21}}^{2} + {2\quad{{Re}\left\lbrack {h_{11}h_{21}^{*}{Xc}^{*}} \right\rbrack}}}} \end{matrix} & (4) \end{matrix}$

When the transmitted signal S₁(t) and S₂(t) are equal to each other and in addition h₁₁=−h₂₁, the strength of received signals is zero, so that the AGC of the receiving apparatus 12 does not function accurately. Furthermore, since Xc in the data interval becomes generally so small as can be regarded as zero, the received power in the data interval becomes |h₁₁−²+|h₂₂|². Hence, the difference in received power between the data interval and the STS interval is 2Re[h₁₁h₂₁*Xc*] as expressed by the third term on the right-hand side of Equation 4. This indicates that even when S₁(t) and S₂(t) are different from each other, the AGC does not function normally if the Xc in the STS interval is large and consequently there is a large difference in power between the STS interval and the data interval. Therefore, an STS different from the STS of the IEEE802.11a standard is required for the MIMO system and the cross-correlation between them needs to be low.

FIG. 4 illustrates a structure of a transmitting apparatus 10. The transmitting apparatus 10 includes a data separating unit 20, a first modulation unit 22 a, a second modulation unit 22 b, . . . and an Nth modulation unit 22 n, which are generically referred to as modulation units 22, a first radio unit 24 a, a second radio unit 24 b, . . . and an Nth radio unit 24 n, which are generically referred to as radio units 24, a control unit 26, and a first transmitting antenna 14 a, a second transmitting antenna 14 b, . . . and an Nth transmitting antenna 14 n, which are generically referred to as transmitting antennas 14 or may be simply called a plurality of antennas 14 or the like. The first modulation unit 22 a includes an error correcting unit 28, an interleave unit 30, a preamble adding unit 32, an IFFT unit 34, a GI unit 36 and a quadrature modulation unit 38. The first radio unit 24 a includes a frequency conversion unit 40 and an amplification unit 42.

The data separating unit 20 separates data to be transmitted into the number of data equal to that of antennas. The error correcting unit 28 performs a coding for error correction on data. The coding to be employed here is a convolutional coding, and the coding rate is to be selected from prescribed values. The interleave unit 30 interleaves data after the convolutional coding. The preamble adding unit 32 adds an STS to the head of a burst signal. Each of preamble adding units 32, which is provided for each of a plurality of transmitting antennas 14, is to store a plurality of STSs to be transmitted in a predetermined period. Although the details of the plurality of STSs will be described later, an STS corresponding to at least one of the plurality of transmitting antennas 14 must use subcarriers at least partially different from those for the STSs corresponding to the other transmitting antennas 14. In other words, it is assumed herein that each of the STSs use the same number of subcarriers, but the subcarriers used are mutually different from the others.

The IFFT unit 34 performs IFFT (Inverse Fast Fourier Transform) in units of FFT point, thereby converting a frequency-domain signal using a plurality of subcarriers into a signal in time domain. The GI unit 36 adds a guard interval to time-domain data. As illustrated in FIG. 3, the guard intervals to be added to the preamble and the data are different from each other. The quadrature modulation unit 38 carries out quadrature modulation. The frequency conversion unit 40 performs a frequency conversion trasnforming a quadrature-modulated signal into a radio-frequency signal. The amplification unit 42 is a power amplifier for amplifying radio-frequency signals. Finally, signals are transmitted from a plurality of transmitting antennas 14. The control unit 26 controls the timing and other functions of the transmitting apparatus 10. It is to be noted that in the present embodiment the transmitting antennas 14 are non-directional and the transmitting apparatus 10 does not perform adaptive array signal processing.

FIG. 5 illustrates a structure of-a receiving apparatus 12. The receiving apparatus 12 includes a first receiving antenna 16 a, a second receiving antenna 16 b, . . . and an Nth receiving antenna 16 n, which are generically referred to as receiving antennas 16, a first radio unit 50 a, a second radio unit 50 b, . . . and an Nth radio unit 50 n, which are generically referred to as radio units 50, a first processing unit 52 a, a second processing unit 52 b, . . . and an Nth processing unit 52 n, which are generically referred to as processing units 52, a first demodulation unit 54 a, a second demodulation unit 54 b, . . . and an Nth demodulation unit 54 n, which are generically referred to as demodulation units 54, a data combining unit 56, and a control unit 58. Signals used include a first radio received signal 200 a, a second radio received signal 200 b, . . . and an Nth radio received signal 200 n, which are generically referred to as radio received signals 200, a first baseband received signal 202 a, a second baseband received signal 202 b, . . . and an Nth baseband received signal 202 n, which are generically referred to as baseband received signals 202, and a first composite signal 204 a, a second composite signal 204 b, . . . and an Nth composite signal 204 n, which are generically referred to as composite signals 204.

The radio units 50 carry out frequency conversion processing of radio received signals 200 of radio frequency into baseband received signals 202 of baseband, and the radio units 50 also carry out amplification processing, A-D conversion processing and the like. Since the communication system 100 assumed herein employs a wireless LAN conforming to the IEEE802.11a standard, the radio frequency of the radio received signals 200 is in the 5 GHz band. The radio units 50 further perform correlation processing for timing detection. The processing units 52 perform adaptive array signal processing on the baseband received signals 202 and output composite signals 204 corresponding to a plurality of transmitted signals. The demodulation units 54 demodulate the composite signals 204. The demodulation units 54 further perform the removal of guard interval, FFT, deinterleave and decoding. The data combining unit 56 combines the signals outputted respectively from the demodulation units 54 in correspondence to the data separating unit 20 as shown in FIG. 4. The control unit 58 controls the timing and other functions of the receiving apparatus 12.

FIG. 6 illustrates a structure of a first radio unit 50 a. The first radio unit 50 a includes an LNA 60, a frequency conversion unit 62, a quadrature detector 64, an AGC 66, an A-D conversion unit 68, and a correlator 70.

The LNA 60 amplifies a first radio received signal 200 a. The frequency conversion unit 62 carries out a frequency conversion between a radio frequency in the 5 GHz band and an intermediate frequency for a signal to be processed. The quadrature detector 64 performs a quadrature detection of an intermediate-frequency signal and generates a baseband analog signal. The AGC 66 controls gain automatically so as to turn the amplitude of a signal into an amplitude within the dynamic range of the A-D conversion unit 68. It is to be noted that, in the initial setting of the AGC 66, the STS in a received signal is used and control is performed in a manner such that the strength of the STS approaches a prescribed value. The A-D conversion unit 68 converts a baseband analog signal into a digital signal and outputs it as a first baseband received signal 202 a.

In order to detect an STS from the first baseband received signal 202 a, the correlator 70 performs a correlation processing on a first baseband received signal 202 a and an STS stored in advance and outputs a correlation value. Although the details will be described later, STS is set for each of the transmitting antennas 14, so that the correlators 70 perform correlation processing for a plurality of STSs and output a plurality of correlation values. The correlation values are inputted to the control unit 58 shown in FIG. 5 via signal lines not shown. The control unit 58 determines the start of receiving a burst signal based on the plurality of correlation values inputted from the plurality of correlators 70 and sends the decision to the processing units 52, the demodulation units 54 and others. Also, in order to demodulate a plurality of signals, the control unit 58 determines the assignment of processing units 52 and demodulation units 54 for the respective signals and send the decision to the processing units 52 and the demodulation units 54.

FIG. 7 illustrates a structure of a first processing unit 52 a. The first processing unit 52 a includes a synthesizing unit 80, a received response vector computing unit 82 and a reference signal storage unit 84. The synthesizing unit 80 includes a first multiplier 86 a, a second multiplier 86 b, . . . and an Nth multiplier 86 n, which are generically referred to as multipliers 86, and an adder 88. Also, signals used include a first received weight signal 206 a, a second received weight signal 206 b, . . . and an Nth received weight signal 206 n, which are generically referred to as received weight signals 206, and a reference signal 208.

The reference signal storage unit 84 stores LTSs.

The received response vector computing unit 82 computes received weight signals 206, as receiving response characteristics of received signals for transmitted signals, from baseband received signals 202 and reference signal 208. Although the method for computing the received weight signals 206 may be arbitrary, one example, as shown below, is based on a correlation processing. It is to be noted that the received weight signals 206 and the reference signal 208 are inputted not only from within the first processing unit 52 a but also from the second processing unit 52 b or the like via signal lines not shown. If the first baseband received signal 202 a is denoted by x₁(t), a second baseband received signal 202 b by x₂(t), the reference signal 208 corresponding to the first transmitting antenna 14 a by S₁(t) and the reference signal 208 corresponding to the second transmitting antenna 14 b by S₂(t), then x₁(t) and x₂(t) will be expressed by the following Equation (5): x ₁(t)=h ₁₁ S ₁(t)+h ₂₁ S ₂(t)x ₂(t) =h ₁₂ S ₁(t)+h ₂₂ S ₂(t)   (5) The noise is ignored here. A first correlation matrix R₁, with E as an ensemble average, is expressed by the following Equation (6): $\begin{matrix} {R_{1} = \begin{bmatrix} {E\left\lbrack {x_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {x_{1}S_{2}^{*}} \right\rbrack} \\ {E\left\lbrack {x_{2}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {x_{2}S_{2}^{*}} \right\rbrack} \end{bmatrix}} & (6) \end{matrix}$

A second correlation matrix R₂ for computing the correlation among the reference signals 208 is given by the following Equation (7): $\begin{matrix} {R_{2} = \begin{bmatrix} {E\left\lbrack {S_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{1}S_{2}^{*}} \right\rbrack} \\ {E\left\lbrack {S_{2}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{2}S_{2}^{*}} \right\rbrack} \end{bmatrix}} & (7) \end{matrix}$

Finally, the first correlation matrix R₁ is multiplied by the inverse matrix of the second correlation matrix R₂ so as to obtain received response vectors, which is expressed by the following Equation (8): $\begin{matrix} {\begin{bmatrix} h_{11} & h_{21} \\ h_{12} & h_{22} \end{bmatrix} = {R_{1}R_{2}^{- 1}}} & (8) \end{matrix}$

The received weight signals are obtained based on received response vectors.

The multipliers 86 weight the baseband received signals 202 with the received weight signals 206 respectively, and the adder 88 adds up the outputs of the multipliers 86 so as to output the composite signal 204.

In terms of hardware, the above-described structure can be realized by a CPU, a memory and other LSIs of an arbitrary computer. In terms of software, it is realized by memory-loaded programs which have a reserved management function or the like, but drawn and described herein are function blocks that are realized in cooperation with those. Thus, it is understood by those skilled in the art that these function blocks can be realized in a variety of forms such as by hardware only, software only or the combination thereof.

FIGS. 8A to 8C show structures of burst formats according to the first embodiment. It is assumed here that the number of transmitting antennas in FIG. 4 is two. FIG. 8A represents a case where two burst signals are transmitted in such a manner as to overlap each other. As mentioned above, a first STS and a second STS are to be separate signal series. On the other hand, a first LTS, a second LTS, a first signal and a second signal are arbitrary signal series, whose explanation is omitted here. In FIG. 8B, of the two burst signals, the first STS and the second STS are transmitted at the same timing, then the first LTS and the first signal are transmitted at a separate timing from the second LTS and the second signal, and then the first data and the second data are transmitted at the same timing again. As mentioned above, the first STS and the second STS are to be separate signal series. On the other hand, the first LTS, the second LTS, the first signal and the second signal, which are transmitted at separate timings here, may be the same signal series, respectively. FIG. 8C illustrates a case where STS is added to one of the signals only. Otherwise, the structure is the same as FIG. 8B.

An STS suited to a MIMO system will be described below, using the same reference numerals as those used in the description of FIG. 3. The cross-correlation between X₁(t) and S₁(t) to be performed at a correlator 70 shown in FIG. 6 is expressed as: $\begin{matrix} {{\sum{{X_{1}(t)}{{S_{1}^{*}(t)}/{sqrt}}\left\{ {\sum{{X_{1}(t)}}^{2}} \right\}{sqrt}\left\{ {{S_{1}(t)}}^{2} \right\}}}\begin{matrix} {= {\sum\left\{ {\left( {{h_{11}{S_{1}(t)}} + {h_{21}{S_{2}(t)}} + {n_{1}(t)}} \right){{S_{1}^{*}(t)}/{sqrt}}\left\{ {\sum{{X_{1}(t)}}^{2}} \right\}} \right\}}} \\ \left. {= {\sum{{\left\{ {{h_{11}{S_{1}(t)}{S_{1}^{*}(t)}} + {h_{21}{S_{2}(t)}{S_{1}^{*}(t)}} + {{S_{1}^{*}(t)}{n_{1}(t)}}} \right)/{sqrt}}\left\{ {\sum{{X_{1}(t)}}^{2}} \right\}}}} \right\} \\ {= \left\{ {{h_{11}{\sum{{S_{1}(t)}{S_{1}^{*}(t)}}}} + {h_{21}{\sum{{S_{1}^{*}(t)}{S_{2}(t)}}}} +} \right.} \\ {{\left. {\sum{S_{1}^{*}(t){n_{1}(t)}}} \right\}/{sqr}}\left\{ {{X_{1}(t)}}^{2} \right\}} \\ {\approx {{\left( {h_{11} + {h_{21}{Xc}}} \right)/{sqrt}}\left\{ {{h_{11}}^{2} + {h_{21}}^{2} + {2{Re}\left\{ {h_{11}h_{21}^{*}{Xc}} \right\}}} \right\}}} \end{matrix}} & (9) \end{matrix}$

Where h₁₁=−h₁₂Xc, the cross-correlation between X₁(t) and S₁(t) is “0”. When Xc is smaller, on the other hand, the equation h₁₁=−h₁₂Xc does not generally hold. In other words, an STS that makes the correlation between S₁(t) and S₂(t) smaller is suited for a MIMO system. The cross-correlation meant here is one for the 12 subcarriers in which STSs are to be placed. There may be cases in such a relationship where the subcarrier numbers for which a plurality of STSs are placed differ. And such a case will be explained below.

FIGS. 9A and 9B illustrate waveforms of known signals transmitted from a transmitting apparatus 10. It is assumed here that the number of transmitting antennas 14 is two. Whereas FIG. 9A shows the STSs to be transmitted from a first transmitting antenna 14 a, FIG. 9B shows the STSs to be transmitted from a second transmitting antenna 14 b. For both FIGS. 9A and 9B, the vertical axis represents “amplitude” and the horizontal axis “FFT point number”. The in-phase (I) component and the quadrature (Q) component of the STS are shown in a distinguishable manner. The STSs illustrated in FIGS. 9A and 9B are expressed in the frequency domain as follows: STS 1 _(−26, 26) =sqrt(13/3){0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,0,0,0}STS 2 _(−26, 26) =sqrt(13/3){0,0,0,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1−j,0,0}  (10)

In other words, a plurality of STSs are selected from among STSs defined in the IEEE802.11a standard. On account of these rules, the cross-correlation between the two STSs becomes zero (0). Moreover, the cross-correlation with the STSs of IEEE802.11a standard as expressed by Equation (1) will be smaller.

FIGS. 10A to 10C illustrate waveforms of known signals transmitted from a transmitting apparatus 10 of FIG. 2. FIGS. 10A to 10C are an extension of FIGS. 9A and 9B to three transmitting antennas 14. The STSs illustrated in FIGS. 10A to 10C are expressed in the frequency domain as follows: STS1 _(−26, 26) =sqrt(13/2){0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0}STS2 _(−26, 26) =sqrt(13/2){0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,}STS3 _(−26, 26) =sqrt(13/2){0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,−1−j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,0,0,0,0,0,0,0,0,0,1+j,0,0,}  (11)

On account of these rules, the cross-correlation between the three STSs becomes zero (0). Moreover, the cross-correlation with the STSs of IEEE802.11a standard as expressed by Equation (1) will be smaller.

FIG. 11 is a flowchart showing a procedure of receiving operation by a receiving apparatus 12. The radio units 50 receive signals, and the AGCs 66 set gains based on STSs contained in the received signals (S10). If the control unit 58 can detect an STS as a result of correlation processing by the correlator 70 (Y of S12), the control unit 58 determines the assignment of processing units 52 and demodulation units 54 for the plurality of signals transmitted (S14). On the other hand, if the control unit 58 cannot detect an STS (N of S12), the procedure returns to step 10. The processing units 52 start adaptive array signal processing by deriving received weight signals 206 based on the LTSs contained in the received signals (S16). The demodulation units 54 start demodulation on the composite signals 204 outputted from the data combining unit 56 (S18).

According to the first embodiment of the present invention, a plurality of known signals are required to use mutually different subcarriers from among a plurality of predetermined subcarriers, so that the cross-correlation between the plurality of known signals can be made smaller. Also, because of this smallness of cross-correlation among a plurality of known signals, the detection accuracy of a plurality of known signals by a receiving apparatus can be improved. Moreover, the smallness of cross-correlation among a plurality of known signals can raise the accuracy with which the AGC is set by a receiving apparatus.

Second Embodiment

A second embodiment of the present invention, as with the first embodiment, relates to preamble signals to be applied to a MIMO system, and aims to reduce the interference between a plurality of preamble signals transmitted. In the first embodiment, the cross-correlation between a plurality of preamble signals is zero (0) as a result of ensuring disagreement between subcarriers to which respective preambles are assigned and using the same number of subcarriers to which respective preambles are assigned. With a transmitting apparatus according to the second embodiment, however, the number of subcarriers to which respective preambles are assigned is not the same; that is, more subcarriers are assigned to a preamble and fewer subccariers are assigned to another preamble. As a result, the autocorrelation value varies from preamble to preamble.

A transmitting apparatus 10 and a receiving apparatus 12 according to the second embodiment are the same as the transmitting apparatus 10 of FIG. 4 and the receiving apparatus 12 of FIG. 5 according to the first embodiment, so that the description thereof is omitted here.

In the first embodiment, the number of subcarriers assigned to each of the STSs is the same. As a result, the cross-correlation becomes small and a higher setting accuracy of AGC is realized. The number of subcarriers per STS becomes smaller, so that the autocorrelation of the STS itself becomes smaller. On the other hand, timing detection by a receiving apparatus 12 and frequency offset estimation (not shown) are carried out based on the autocorrelation of STS, so that the higher the autocorrelation of STS, the higher the detection accuracy and estimation accuracy will generally be. That is, in one mode of the second embodiment, the autocorrelation characteristics of STS corresponding to one of the plurality of transmitting antennas 14 shown in FIG. 4 are set higher than those of STSs corresponding to the other transmitting antennas 14. Also, the number of subcarriers to be used for the STS corresponding to one of the plurality of transmitting antennas 14 is set larger than the number of subcarriers to be used for the STSs corresponding to the other transmitting antennas 14.

To be more specific, let us assume here that a transmitting apparatus 10 is provided with three transmitting antennas 14. When signals are to be transmitted by a first transmitting antenna 14 a and a second transmitting antenna 14 b only, six subcarriers are assigned to the STS corresponding to the first transmitting antenna 14 a and six subcarriers are assigned to the STS corresponding to the second transmitting antenna 14 b. On the other hand, when signals are to be transmitted by the three transmitting antennas 14, six subcarriers are assigned to the STS corresponding to the first transmitting antenna 14 a, three subcarriers are assigned to the STS corresponding to the second transmitting antenna 14 b, and three subcarriers are assigned to the STS corresponding to the third transmitting antenna 14 c. Consequently, a receiving apparatus 12 as shown in FIG. 5 sets the AGCs based on the respective STSs. The receiving apparatus 12 performs timing detection and frequency offset estimation based on the STS with the largest number of subcarriers among the plurality of STSs.

In another mode of the second embodiment, subcarriers used by different STSs are partially overlapped. Compared with the embodiments so far described, the cross-correlation becomes larger, but the autocorrelation, too, becomes larger. That is, if the number of subcarriers used only for the STS corresponding to one of a plurality of transmitting antennas 14 is called a first value and that of subcarriers used only for the STSs corresponding to the other transmitting antennas 14 is called a second value, out of the plurality of subcarriers used for the STSs defined by the IEEE802.11a standard, then the plurality of STSs will be determined in a manner such that the first value is larger than the second value. For example, eight subcarriers are assigned to the STS corresponding to the first transmitting antenna 14 a, six subcarriers are assigned to the STS corresponding to the second transmitting antenna 14 b, and two subcarriers of them are overlapped with each other.

Moreover, when the number of transmitting antennas is three, the above-mentioned second value is assumed to be the number of subcarriers used only for the STS corresponding to one of the other transmitting antennas 14. In such a case, the cross-correlation characteristics between the second transmitting antenna 14 b and the third transmitting antenna 14 c in relation to the STS corresponding to the first transmitting antenna 14 a are so set as to be lower than the cross-correlation characteristics between the STS corresponding to the second transmitting antenna 14 b and the STS corresponding to the third transmitting antenna 14 c. That is, while all the STSs corresponding to the three transmitting antennas 14 use six subcarriers each, one of them uses four subcarriers exclusively, another of them uses two subcarriers exclusively, and still another of them uses none of the subcarriers exclusively.

Furthermore, the subcarriers may be assigned so that the aforementioned second value is “0”. That is, six subcarriers are assigned exclusively to the STS corresponding to the first transmitting antenna 14 a, and the remaining six subcarriers are shared by the STSs corresponding respectively to the second transmitting antenna 14 b and the third transmitting antenna 14 c. In such a case, those STSs use such signal series as to make the cross-correlation smaller.

According to the second embodiment of the present invention, the number of subcarriers to be used by each of a plurality of known signals, from among predetermined subcarriers, is made to vary and differ within the plurality of known signals, so that the plurality of known signals can be so designed as to produce predetermined values for the autocorrelation or the cross-correlation. Moreover, the value of autocorrelation for predetermined known signals is made larger, so that the timing detection accuracy and the frequency offset estimation accuracy at a receiving apparatus can be improved.

Third Embodiment

A third embodiment of the present invention relates to a MIMO system as with the first embodiment. However, the third embodiment concerns correlation processing at a receiving apparatus. As described above, when a plurality of known signals are transmitted in parallel from a plurality of antennas of a transmitting apparatus, a receiving apparatus needs to have a plurality of correlators corresponding respectively to the plurality of known signals in order to detect the timing from the plurality of received known signals. Now the use of a plurality of correlators enlarges the scale of circuitry of the receiving apparatus. A MIMO system according to the third embodiment defines a predetermined relationship between a plurality of series of known signals in the time domain (hereinafter referred to as “time-domain known signals”). The transmitting apparatus transmits a plurality of time-domain known signals as defined above, and the receiving apparatus reduces the amount of correlation processing based on the relationship among the plurality of time-domain known signals. That is, the correlation processing, which is generally done by multiplication and addition, is performed in such a manner here that two correlation values are outputted by arranging the multiplication into a common use in the correlation processings on the two series of known signals and adding the results of multiplication in different combinations.

A transmitting apparatus 10 and a receiving apparatus 12 according to the third embodiment are the same as the transmitting apparatus 10 of FIG. 4 and the receiving apparatus 12 of FIG. 5 according to the first embodiment, so that the description thereof is omitted here.

FIGS. 12A and 12B illustrate waveforms of known signals transmitted from a transmitting apparatus 10 according to the third embodiment. It is assumed here that the number of transmitting antennas is three and that the STS corresponding to a first transmitting antenna 14 a is placed at subcarrier numbers “−24, −16, −12, −8, −4, 4, 8, 12, 16, 24”, the STS corresponding to a second transmitting antenna 14 b is placed at subcarrier number “20”, and the STS corresponding to a third transmitting antenna 14 c is placed at subcarrier number “−20”. Whereas FIG. 12A shows a waveform of the STS corresponding to the second transmitting antenna 14 b, FIG. 12B shows a waveform of the STS corresponding to the third transmitting antenna 14 c. There is a relationship in which the values of the in-phase component of the waveform of the STS corresponding to the second transmitting antenna 14 b is equal to the values of the quadrature component of the waveform of the STS corresponding to the third transmitting antenna 14 c and the values of the quadrature component of the waveform of the STS corresponding to the second transmitting antenna 14 b is equal to the values of the in-phase component of the waveform of the STS corresponding to the third transmitting antenna 14 c.

FIG. 13 illustrates a structure of a correlator 70 according to the third embodiment. The correlator 70 includes a first I delay unit 300 a, a second I delay unit 300 b and a third I delay unit 300 c, which are generically referred to as I delay units 300, a first Q delay unit 302 a, a second Q delay unit 302 b and a third Q delay unit 302 c, which are generically referred to as Q delay units 302, a first I storage unit 304 a, a second I storage unit 304 b, a third I storage unit 304 c and a fourth I storage unit 304 d, which are generically referred to as I storage units 304, a first Q storage unit 306 a, a second Q storage unit 306 b, a third Q storage unit 306 c and a fourth Q storage unit 306 d, which are generically referred to as Q storage units 306, a first multiplier 308 a, a second multiplier 308 b, a third multiplier 308 c, a fourth multiplier 308 d, a fifth multiplier 308 e, a sixth multiplier 308 f, a seventh multiplier 308 g, an eighth multiplier 308 h, a ninth multiplier 308 i, a tenth multiplier 308 j, an eleventh multiplier 308 k, a twelfth multiplier 308 l, a thirteenth multiplier 308 m, a fourteenth multiplier 308 n, a fifteenth multiplier 308 o and a sixteenth multiplier 308 p, which are generically referred to as multipliers 308, a first adder 310 a, a second adder 310 b, a third adder 310 c, a fourth adder 310 d, a fifth adder 310 e, a sixth adder 310 f, a seventh adder 310 g, an eighth adder 310 h, a ninth adder 310 i, a tenth adder 310 j, an eleventh adder 310 k, a twelfth adder 310 l, a thirteenth adder 310 m, a fourteenth adder 310 n, a fifteenth adder 310 o and a sixteenth adder 310 p, which are generically referred to as adders 310, and a first adder 312 a, a second adder 312 b, a third adder 312 c and a fourth adder 312 d, which are generically referred to as adders 312. The signals used include a first correlation in-phase value 210, a first correlation quadrature value 212, a second correlation in-phase value 214 and a second correlation quadrature value 216.

A first baseband received signal 202 a outputted from an A-D conversion unit 68 shown in FIG. 6 is inputted to the correlator 70. In FIG. 6, the signal line for transmitting the first baseband received signal 202 a is represented by a single straight line. In fact, however, the first baseband received signal 202 a is a signal containing an in-phase component and a quadrature component. Also, in order to simplify the explanation and drawing here, there are only three each of the I delay unit 300 and the Q delay unit 302 and correlation processings are carried out based on four first baseband received signals 202 a. However, it is assumed that in practice, correlation processings are performed on 16 baseband received signals 202 a or a number other than 16 of the first baseband received signal 202 a. Furthermore, the correlator 70 also performs correlation processing for the STS corresponding to a first transmitting antenna 14 a as described earlier and is provided with a circuitry therefor, but the description thereof will be omitted here.

The I delay units 300 and the Q delay units 302 delay in succession the value of the in-phase component and the value of the quadrature component, respectively, of an inputted first baseband received signal 202 a. The I storage units 304 and the Q storage units 306 store each component of the waveform of the STS corresponding to the first transmitting antenna 14 a, namely, the STSs converted into the time domain (hereinafter referred to “time-domain STSs”, which may be used in the same sense as the aforementioned “time-domain known signals”). The I storage units 304 and the Q storage units 306 store the in-phase component and the quadrature component, respectively, of the time-domain STS.

The multipliers 308 carry out multiplications of correlation processing. That is, the first multiplier 308 a multiplies the value of the quadrature component of a first baseband received signal 202 a by the value of the in-phase component of a time-domain STS; the second multiplier 308 b multiplies the value of the in-phase component of the first baseband received signal 202 a by the value of the in-phase component of the time-domain STS; the third multiplier 308 c multiplies the value of the quadrature component of the first baseband received signal 202 a by the value of the quadrature component of the time-domain STS; and the fourth multiplier 308 d multiplies the value of the in-phase component of the first baseband received signal 202 a by the value of the quadrature component of the time-domain STS. And the fifth multiplier 308 e, the ninth multiplier 308 i and the thirteenth multiplier 308 m correspond to the first multiplier 308 a; the sixth multiplier 308 f, the tenth multiplier 308 j and the fourteenth multiplier 308 n correspond to the second multiplier 308 b; the seventh multiplier 308 g, the eleventh multiplier 308 k and the fifteenth multiplier 308 o correspond to the third multiplier 308 c; and the eighth multiplier 308 h, the twelfth multiplier 308 l and the sixteenth multiplier 308 p correspond to the fourth multiplier 308 d. In other words, sharing is done for the multiplications of the two time-domain STSs.

The adders 310 add the results of the multiplications outputted from the multipliers 308, and the results of these additions are further added by the adders 312. As a result, correlation values for the two time-domain STSs are generated in such a form as to have the in-phase component and the quadrature component, respectively. The first adder 310 a subtracts the result of multiplication by the fourth multiplier 308 d from the result of multiplication by the first multiplier 308 a; the second adder 310 b adds the result of multiplication by the second multiplier 308 b and the result of multiplication by the third multiplier 308 c together; the third adder 310 c adds the result of multiplication by the second multiplier 308 b and the result of multiplication by the third multiplier 308 c together; and the fourth adder 310 d subtracts the result of multiplication by the fourth multiplier 308 d from the result of multiplication by the first multiplier 308 a. And the fifth adder 310 e, the ninth adder 310 i and the thirteenth adder 310 m correspond to the first adder 310 a; the sixth adder 310 f, the tenth adder 310 j and the fourteenth adder 310 n correspond to the second adder 310 b; the seventh adder 310 g, the eleventh adder 310 k and the fifteenth adder 310 o correspond to the third adder 310 c; and the eighth adder 310 h, the twelfth adder 310 l and the sixteenth adder 310 p correspond to the fourth adder 310 d.

The first adder 312 a outputs a first correlation quadrature value 212 by computing the quadrature component of the correlation value for one of the time-domain STS; the second adder 312 b outputs a first correlation in-phase value 210 by computing the in-phase component of the correlation value for one of the time-domain STS; the third adder 312 c outputs a second correlation quadrature value 216 by computing the oquadrature component of the correlation value for the other of the time-domain STS; and the fourth adder 312 d outputs a second correlation in-phase value 214 by computing the in-phase component of the correlation value for the other of the time-domain STS. Here, the first adder 312 a adds up the results of addition by the fourth adder 310 d, the eighth adder 310 h, the twelfth adder 310 l and the sixteenth adder 310 p; the second adder 312 b adds up the results of addition by the third adder 310 c, the seventh adder 310 g, the eleventh adder 310 k and the fifteenth adder 310 o; the third adder 312 c adds up the results of addition by the second adder 310 b, the sixth adder 310 f, the tenth adder 310 j and the fourteenth adder 310 n; and the fourth adder 312 d adds up the results of addition by the first adder 310 a, the fifth adder 310 e, the ninth adder 310 i and the thirteenth adder 310 m.

FIGS. 14A and 14B illustrate waveforms of known signals transmitted from a transmitting apparatus 10 according to the third embodiment. In FIGS. 14A and 14B, just as well as in FIGS. 12A and 12B, there is shown a relationship in which the values of the in-phase component of the time-domain STS corresponding to the second transmitting antenna 14 b is equal to the values of the quadrature component of the time-domain STS corresponding to the third transmitting antenna 14 c and the values of the quadrature component of the time-domain STS corresponding to the second transmitting antenna 14 b is equal to the values of the in-phase component of the time-domain STS corresponding to the third transmitting antenna 14 c. Accordingly, a correlator 70 as shown in FIG. 13 proves effective as a circuit for performing correlation processing for these signals.

According to the third embodiment of the present invention, the circuit may be reduced in scale because the delay units for delaying inputted signals, the storage units for storing reference signals and the multipliers to be used in correlation processing may be shared in the processing for a plurality of reference signals.

Fourth Embodiment

A fourth embodiment of the present invention, as with the third embodiment, relates to correlation processings at a receiving apparatus. Of a plurality time-domain known signals according to the fourth embodiment, the variable period of one of the waveforms is twice the variable period of the other. Moreover, the time-domain known signals of longer variable period only are stored. The correlation processing for the time-domain known signals not stored is executed after selecting a predetermined value from among the values of the stored time-domain known signals. Hence, part of the correlation processings for the two time-domain known signals can be shared.

A transmitting apparatus 10 and a receiving apparatus 12 according to the fourth embodiment are the same as the transmitting apparatus 10 of FIG. 4 and the receiving apparatus 12 of FIG. 5 according to the first embodiment, so that the description thereof is omitted here. According to the fourth embodiment, two time-domain STSs, of which the variable period of one time-domain STS is ½ of that of the other time-domain STS, are transmitted from two transmitting antennas 14. Let us call here the time-domain STS with longer variable period a “first time-domain STS” and the time-domain STS with shorter variable period a “second time-domain STS”. Let us also assume here that the period of the second time-domain STS is ½ of the period of the first time-domain STS.

FIG. 15 illustrates a structure of a correlator 70 according to the fourth embodiment. Compared with the correlator 70 shown in FIG. 13, the correlator 70 shown in FIG. 15 further includes a first multiplier 314 a, a second multiplier 314 b, a third multiplier 314 c, a fourth multiplier 314 d, a fifth multiplier 314 e, a sixth multiplier 314 f, a seventh multiplier 314 g, an eighth multiplier 314 h, a ninth multiplier 314 i, a tenth multiplier 314 j, an eleventh multiplier 314 k, a twelfth multiplier 314 l, a thirteenth multiplier 314 m, a fourteenth multiplier 314 n, a fifteenth multiplier 314 o and a sixteenth multiplier 314 p, which are generically referred to as multipliers 314, and a first adder 316 a, a second adder 316 b, a third adder 316 c, a fourth adder 316 d, a fifth adder 316 e, a sixth adder 316 f, a seventh adder 316 g and an eighth adder 316 h, which are generically referred to as adders 316.

The I storage units 304 and the Q storage units 306 store first time-domain STS. Here, the processings up to the output of a second correlation in-phase value 214 and a second correlation quadrature value 216 are the same as the processings up to the output of a second correlation in-phase value 214 and a second correlation quadrature value 216 in FIG. 13, so that the description thereof is omitted here.

In the processings up to the output of a first correlation in-phase value 210 and a second correlation quadrature value 216, the multipliers 314 multiply the delayed first baseband received signals 202 a by the values from the I storage units 304 and the values from the Q storage units 306. However, of the I storage units 304, the first I storage unit 304 a and the third I storage unit 304 c only are used. That is, the multipliers 314, which carry out multiplications using the I storage units 304, are arranged in a time series of the second multiplier 314 b, the sixth multiplier 314 f, the tenth multiplier 314 j and the fourteenth multiplier 314 n. The second multiplier 314 b performs multiplication using the first I storage unit 304 a, but the sixth multiplier 314 f performs multiplication using the third I storage unit 304 c instead of the second I storage unit 304 b. In other words, values of the I storage units 304 are each selected as every other integer, that is, at intervals of “2”, a reciprocal of “½”, which is the ratio of the period of the second time-domain STS to that of the first time-domain STS. The same applies to the Q storage units 306.

According to the fourth embodiment of the present invention, the circuit can be reduced in scale because the delay units for delaying inputted signals and the storage units for storing reference signals can be shared in the processing for a plurality of reference signals.

Fifth Embodiment

A fifth embodiment of the present invention, as with the third embodiment, relates to correlation processings at a receiving apparatus. Of a plurality time-domain known signals according to the fifth embodiment, one has a waveform with which either of the in-phase component and the quadrature component is zero (“0”) and in addition the amplitude is constant. And the other of the time-domain known signals has a waveform which is inverted from the quadrature component of the former. A receiving apparatus which receives these time-domain known signals can not only share part of the correlation processings for the two time-domain known signals but also has no need for multipications.

FIG. 16 illustrates a waveform of a known signal transmitted from a transmitting apparatus 10 according to the fifth embodiment. It is assumed here that the number of transmitting antennas 14 is three and that the STS corresponding to a first transmitting antenna 14 a is placed at subcarrier numbers “−24, −20, −12, −8, −4, 4, 8, 12, 20, 24”, the STS corresponding to a second transmitting antenna 14 b is placed at subcarrier number “−16”, and the STS corresponding to a third transmitting antenna 14 c is placed at subcarrier number “16”. FIG. 16 shows a time-domain STS for the STS corresponding to the second transmitting antenna 14 b. On the other hand, a time-domain STS for the STS corresponding to the third transmitting antenna 14 c has a waveform which has inverted the quadrature component of FIG. 16.

FIG. 17 illustrates a structure of a correlator 70 according to the fifth embodiment. Compared to the correlator 70 shown in FIG. 13, the correlator 70 shown in FIG. 17 further includes a first inversion unit 318 a, a second inversion unit 318 b, a third inversion unit 318 c, a fourth inversion unit 318 d, a fifth inversion unit 318 e and a sixth inversion unit 318 f, which are generically referred to as inversion units 318.

The inversion units 318 invert the values of inputted signals. That is, the inversion units 318 convert positive values to negative values and vice versa.

The adders 312 add up a first baseband received signal 202 a, output signals from the I delay units 300 and output signals from the inversion units 318, and outputs a first correlation in-phase value 210, a first correlation quadrature value 212, a second correlation in-phase value 214 and a second correlation quadrature value 216, respectively.

According to the fifth embodiment of the present invention, the circuit can be reduced in scale because the delay units for delaying inputted signals can be shared in the processing for a plurality of reference signals and furthermore the multipliers can be eliminated in the correlation processing.

Sixth Embodiment

A sixth embodiment of the present invention, as with the other above-described embodiments, relates to a MIMO system. In a MIMO system, signals independent from each other are transmitted from a plurality of transmitting antennas, as described earlier. And, in the embodiments described heretofore, the number of transmitting antennas is fixed. However, there may be cases where the number of transmitting antennas is to be changed according to the volume of data to be transmitted and other factors. That is, the number of transmitting antennas is decreased when the volume of data to be transmitted is small and increased when the volume of data to be transmitted is large. When a transmitting apparatus changes the number of antennas for transmitting signals as appropriate, it is necessary for a receiving apparatus to recognize the changed number of transmitting antennas if it is to receive data with accuracy. However, if a transmitting apparatus is communicating the number of antennas transmitting data to a receiving apparatus by means of a predetermined control signal, then the control signal will have an effect of lowering the data transmission efficiency. Thus, it is preferred that a receiving apparatus can recognize the number of antennas transmitting data even without the control signal.

According to the sixth embodiment, a transmitting apparatus changes the pattern of STS with the number of antennas transmitting data, and a receiving apparatus recognizes the number of antennas transmitting data by detecting the changed pattern of STS. More specifically, a plurality of transmitting antennas are divided into one (hereinafter referred to as “main antenna”) and the rest (hereinafter referred to as “sub-antennas”), and the pattern of STS transmitted from the main antenna is changed according to the number of transmitting antennas to be used. Also, to ensure the accurate receiving of any changed pattern of STS transmitted from the main antenna, the STS to be transmitted by the main antenna and the STSs to be transmitted by the sub-antennas use different subcarriers. That is, the respective STSs are so determined as to make the value of cross-correlation between them smaller.

A receiving apparatus according to the sixth embodiment, as described above, detects an STS pattern transmitted from the main antenna. This detection is accomplished by a correlation processing between a signal received and an STS pattern stored beforehand. For example, when the number of transmitting antennas is two and three, a correlator is necessary that can respond to both the STS patterns. In the sixth embodiment, the patterns of STSs transmitted from the main antenna when the number of transmitting antennas is two and three are so defined as to be in a relationship of inverted phases between each other as in the third embodiment. Therefore, if it is provided with a correlator as described in the third embodiment, the receiving apparatus can detect change in the pattern of STS sent from the main antenna. Moreover, it is not necessary to have correlators corresponding to the respective STS patterns.

A transmitting apparatus according to the sixth embodiment is the same type as the transmitting apparatus 10 of FIG. 4. The transmitting apparatus 10 is provided with a determination unit which is not shown. The determination unit determines the number of transmitting antennas 14 that are to transmit signals, out of N units of transmitting antennas 14, according to predetermined instructions. It is to be noted here that the predetermined instructions may be arbitrary. For example, they may be received from an application for transmitting data in a communication system 100 according to the type of application or the amount of data. Or a measurement unit (not shown), which measures the quality of the channel between a transmitting apparatus 10 and a receiving apparatus 12, may input the instructions according to the measurement results to the determination unit. Moreover, modulation units 22 and radio units 24, corresponding to the number of transmitting antennas 14 as determined by the transmitting antennas 14, operate to send signals.

A preamble adding unit 32, as described above, stores STSs and LTSs beforehand and adds STS and LTS at the head of a burst signal. Moreover, the preamble adding unit 32 selects a predetermined STS from among a plurality of STSs stored beforehand, according to the number of transmitting antennas determined by the determination unit. For example, a preamble adding unit 32 contained in a modulation unit 22 corresponding to a main antenna selects an STS corresponding to the main antenna. STSs corresponding to a main antenna are stored according to the number of transmitting antennas 14 that can be determined, and an STS corresponding to the number of transmitting antennas 14 that has been determined by a determination unit is selected. For example, where it is possible that “2” or “3” is determined as the number of transmitting antennas 14, the preamble adding unit 32 stores STSs corresponding to the “2” as the number of transmitting antennas 14 and STSs corresponding to the “3” as the number of transmitting antennas 14. And when the determination unit determines “2” as the number of transmitting antennas 14, the preamble adding unit 32 will select STS corresponding to the “2” and adds the selected STS to the burst signal. On the other hand, one or more preamble adding units 32 contained in one or more modulation units 22 corresponding to sub-antennas select STSs corresponding to the sub-antennas. Where there are a plurality of sub-antennas, the patterns of STSs corresponding thereto differ from each other so as to reduce mutual interference.

FIG. 18 is a table showing a relationship between the number of transmitting antennas 14 for transmitting data and the patterns of STSs transmitted by the transmitting antennas, according to the sixth embodiment. The table shows the numbers of transmitting antennas 14 vertically. The table shows the transmitting antennas 14 to be used horizontally and the STSs corresponding thereto in relation to the numbers of antennas 14. Specifically, when the number of transmitting antennas 14 is “1”, an STS defined by the aforementioned IEEE802.11a standard (hereinafter referred to as “Legacy STS”) is transmitted from the first transmitting antenna 14 a. When the number of transmitting antennas 14 is “2”, “STS1” is transmitted from the first transmitting antenna 14 a and “STSa” is transmitted from the second transmitting antenna 14 b. When the number of transmitting antennas 14 is “3”, “STS1” is transmitted from the first transmitting antenna 14 a, “STS2” is transmitted from the second transmitting antenna 14 b and “STSb” is transmitted from the third transmitting antenna 14 c. Here, the second transmitting antenna 14 b when the number of transmitting antennas 14 is “2” and the third transmitting antenna 14 c when the number of transmitting antennas 14 is “3” are the main antenna, and the rest are sub-antennas.

In relation to the description heretofore, the STS corresponding to the main antenna when the number of transmitting antennas 14 is “2” is “STSa”, and the STS corresponding to the main antenna when the number of transmitting antennas 14 is “3” is “STSb”. On the other hand, the STS corresponding to the sub-antenna when the number of transmitting antennas 14 is “2” is “STS1”, and the STSs corresponding to the sub-antennas when the number of transmitting antennas 14 is “3” are “STS1” and “STS2”. For the simplicity of explanation, “STSa” and “STSb” will be generically referred to as “STS for main antenna”, and “STS1” and “STS2” will be generically referred to as “STS for sub-antennas”. It is to be noted also that the number of transmitting antennas 14 for transmitting signals has thus far been described as “2” or “3”, but it may be a number other than these.

It is to be noted that the relationship between these STSs and Legacy STS is so defined that the plurality of carriers to be used for the STSs for main antenna and the STSs for sub-antenna, respectively, correspond to any of the 12 subcarriers used for Legacy STS. In this case, the STSs for main antenna and the STSs for sub-antenna use six mutually different subcarriers of the 12 subcarriers for Legacy STS. As a result of this definition, the cross-correlation value between the STSs for main antenna and the STSs for sub-antenna is zero (“0”). It is assumed that the six subcarriers used by the STSs for main antenna are fixed and not dependent on the number of transmitting antennas 14 for transmitting data and the six subcarriers used by the STSs for sub-antennas are fixed and not dependent on the number of transmitting antennas 14 for transmitting data. Accordingly, where there are a plurality of sub-antennas and if, for instance, the number of transmitting antennas 14 is three, “STS1” and “STS2” will use the same six subcarriers.

The STSs for main antenna include “STSa” and “STSb”, but because of pattern difference between STSa and STSb, they have a function of conveying the number of transmitting antennas 14 transmitting signals to a receiving apparatus 12. Hence, these STSs need to be different from each other to such an extent that the receiving apparatus 12 can identify the STSa and STSb from the signals it has received. In other words, when the number of transmitting antennas 14 transmitting signals differs, the STSs for main antenna are so defined as to become those of different main antenna, or more specifically defined by values that make the cross-correlation characteristics between STSa and STSb smaller. Specific values for these will be mentioned later.

On the other hand, the STSs for sub-antenna use the same subcarriers especially when there are a plurality of sub-antennas, so that “STS1” and “STS2” are defined with such patterns as will reduce interference between these STSs. The STSs for sub-antenna are defined by values that make the cross-correlation characteristics smaller. Thus, even when the number of transmitting antennas 14 used to transmit signals increases, the number of subcarriers used for the STSs for main antenna only remains fixed at “6”, but the number of subcarriers used for one STS for sub-antenna only reduces to zero (“0”).

FIG. 19 illustrates a waveform of STSa, and FIG. 20 a waveform of STSb. These represent the values of STSs for main antenna in the time domain when the number of antennas transmitting signals is “2” and “3”. The STSs for main antenna have both the in-phase component and quadrature component in the time domain and are so defined that for the two kinds of number of antennas to transmit signals, namely, “2” and “3”, the value of the in-phase component of “STSa” is equal to the value of the quadrature component of “STSb” and the value of the quadrature component of “STSa” is equal to the value of the in-phase component of “STSb”. On the other hand, “STSa” is expressed in the frequency domain as follows: $\begin{matrix} \begin{matrix} {S_{{- 26},26} = {{{sqrt}\left( \frac{26}{6} \right)}\left\{ {0,0,0,0,0,0,{1 + j},} \right.}} \\ {0,0,0,{1 + j},0,0,0,0,0,0,0,0,0,0,} \\ {0,{{- 1} - j},0,0,0,0,0,0,0,{1 + j},0,0,} \\ {0,0,0,0,0,0,0,0,0,{{- 1} - j},0,0,0,} \\ \left. {{1 + j},0,0,0,0,0,0} \right\} \end{matrix} & (12) \end{matrix}$

“STSb” is expressed in the frequency domain as follows: $\begin{matrix} \begin{matrix} {S_{{- 26},26} = {{{sqrt}\left( \frac{26}{6} \right)}\left\{ {0,0,0,0,0,0,{1 + j},} \right.}} \\ {0,0,0,{{- 1} - j},0,0,0,0,0,0,0,0,0,0,} \\ {0,{1 + j},0,0,0,0,0,0,0,{{- 1} - j},0,0,0,} \\ {0,0,0,0,0,0,0,0,{1 + j},0,0,0,{1 + j},} \\ \left. {0,0,0,0,0,0} \right\} \end{matrix} & (13) \end{matrix}$

FIG. 21 illustrates a waveform of STS1, and FIG. 22 a waveform of STS2. These represent the values of STSs for sub-antenna in the time domain. “STS1” is expressed in the frequency domain as follows: $\begin{matrix} \begin{matrix} {S_{{- 26},26} = {{{sqrt}\left( \frac{26}{6} \right)}\left\{ {0,0,{{- 1} - j},0,0,0,0,0,0,} \right.}} \\ {0,0,0,0,0,{{- 1} - j},0,0,0,{{- 1} - j},0,0,0,} \\ {0,0,0,0,0,0,0,0,0,0,0,0,{{- 1} - j},0,} \\ {0,0,{1 + j},0,0,0,0,0,0,0,0,0,0,0,} \\ \left. {{1 + j},0,0} \right\} \end{matrix} & (14) \end{matrix}$

“STS2” is expressed in the frequency domain as follows: $\begin{matrix} \begin{matrix} {S_{{- 26},26} = {{{sqrt}\left( \frac{26}{6} \right)}\left\{ {0,0,{{- 1} - j},0,0,0,0,0,0,} \right.}} \\ {0,0,0,0,0,{1 + j},0,0,0,{1 + j},0,0,0,} \\ {0,0,0,0,0,0,0,0,0,0,0,0,{{- 1} - j},} \\ {0,0,0,{1 + j},0,0,0,0,0,0,0,0,0,0,} \\ \left. {0,{1 + j},0,0} \right\} \end{matrix} & (15) \end{matrix}$

A receiving apparatus 12, a first radio unit 50 a, a first processing unit 52 a and a correlator 70 according to the sixth embodiment are of the same type as a receiving apparatus 12 of FIG. 5, a first radio unit 50 a of FIG. 6, a first processing unit 52 a of FIG. 7 and a correlator 70 of FIG. 13.

The radio units 50 receive signals transmitted respectively from a plurality of transmitting antennas 14. The correlator 70 detects STSs from a signal received. The description thereof hereunder, however, covers specifically an operation of detecting STSs for main antenna. The correlator 70 stores the values of “STSa” in the I storage units 304 and the Q storage units 306. The correlator 70 carries out correlation between received signals and stored values by the multipliers 308 and the adders 310 and outputs the correlation values between the received signal and “STSa” as a first correlation in-phase value 210 and a first correlation quadrature value 212 and the correlation values between the received signal and “STSb” as a second correlation in-phase value 214 and a second correlation quadrature value 216.

An estimation unit, which is not shown here, inputs a first correlation in-phase value 210, a first correlation quadrature value 212, a second correlation in-phase value 214 and a second correlation quadrature value 216 and derives a magnitude calculated from the first correlation in-phase value 210 and the first correlation quadrature value 212 (hereinafter referred to as “first magnitude”) and a magnitude calculated from the second correlation in-phase value 214 and the second correlation quadrature value 216 (hereinafter referred to as “second magnitude”). Furthermore, if the first magnitude is larger than the second magnitude, the estimation unit estimates the transmitted STS for main antenna to be “STSa” and determines that the number of transmitting antennas 14 transmitting signals is “2”. On the other hand, if the first magnitude is not larger than the second magnitude, the estimation unit estimates the transmitted STS for main antenna to be “STSb” and determines that the number of transmitting antennas 14 transmitting signals is “3”. And according to the number of transmitting antennas 14 thus determined, the receiving apparatus 12 performs settings for demodulation. That is, the receiving apparatus 12 will activate the first demodulation unit 54 a and the second demodulation unit 54 b if the number of transmitting antennas 14 is “2” or the first demodulation unit 54 a through the third demodulation unit 54 c if the number of transmitting antennas 14 is “3”.

FIG. 23 is a flowchart showing a procedure of a receiving operation by a receiving apparatus 12. Radio units 50 receive signals, and AGC units 66 set AGCs based on the STSs contained in the received signals (S50). Correlators 70 perform correlation processing and thereby detects STSs (S52). And if the STS detected is “STSa” (Y of S54), an estimation unit will decide on the receiving of two-series signals (S56) and activate a first demodulation unit 54 a and a second demodulation unit 54 b. On the other hand, if the STS detected is not “STSa” but “STSb” (N of S54), the estimation unit will decide on the receiving of three-series signals (S58) and activate a first demodulation unit 54 a through a third demodulation unit 54 c. Processing units 52 start adaptive array signal processing by deriving received weight signals 206 based on the LTSs contained in the received signals (S60). Demodulation units 54 start demodulation on the composite signals 204 outputted from a data combining unit 56 (S62).

According to the sixth embodiment of the present invention, a receiving apparatus can recognize the number of antennas transmitting signals without being notified about the number of antennas transmitting the signals from a transmitting apparatus. Also, the circuit can be reduced in scale because the delay units for delaying inputted signals, the storage units for storing reference signals and the multipliers to be used in correlation processing can be shared in the processing for two STSs.

Seventh Embodiment

A seventh embodiment of the present invention, as with the sixth embodiment, relates to a technology for a MIMO system, in which a transmitting apparatus changes the pattern of STS with the number of antennas transmitting data and a receiving apparatus recognizes the number of antennas transmitting data by detecting the changed pattern of STS. Also, as with the sixth embodiment, the patterns of STSs transmitted from a main antenna are given a predetermined relationship according to the number of transmitting antennas transmitting data. The receiving apparatus detects a plurality of correlation values with a single correlator by making use of this relationship. In the sixth embodiment, the aforementioned predetermined relationship is so defined as to have mutually inverted phases, but in the seventh embodiment, the relationship is defined in a manner such that the absolute values of the respective components are exchanged and the signs are inverted.

FIG. 24 is a table showing a relationship between the number of transmitting antennas 14 for transmitting data and the patterns of STSs transmitted by the transmitting antennas, according to the seventh embodiment. As with FIG. 18, the table shows the numbers of transmitting antennas 14 vertically and the transmitting antennas 14 to be used horizontally and the STSs corresponding thereto in relation to the numbers of antennas 14. In FIG. 24, the STS transmitted from the third transmitting antenna 14 c for the number “3” of transmitting antennas 14 transmitting signals, namely, the STS for main antenna, is “STSb′”. Here, the “STSb′” is expressed in the frequency domain as follows: $\begin{matrix} {S_{{- 26},26} = {{{sqrt}\left( \frac{26}{6} \right)}\left\{ {0,0,0,0,0,0,{{- 1} - j},0,0,0,{1 + \quad j},0,0,0,0,0,0,0,0,0,0,{0 - 1 - \quad j},0,0,0,0,0,0,0,0,{1 + \quad j},0,0,0,0,0,0,0,0,0,0,0,{{- 1} - \quad j},0,0,0,{{- 1} - j},0,0,0,0,0,0} \right\}}} & (16) \end{matrix}$

Otherwise, the seventh embodiment is the same as the sixth embodiment, so that the description thereof is omitted here.

FIG. 25 illustrates a waveform of STSb′. STSb′ has inverted sign in relation to STSb. STSb′ has the in-phase component and quadrature component in the time domain and is so defined that for the two kinds of number of antennas to transmit signals, namely, “2” and “3”, the absolute value of the in-phase component of “STSa” is equal to the absolute value of the quadrature component of “STSb′”, which has inverted sign, and the absolute value of the quadrature component of “STSa” is equal to the absolute value of the in-phase component of “STSb′”, which has inverted sign.

FIG. 26 illustrates a structure of a correlator 70 according to the seventh embodiment. Compared to the correlator 70 shown in FIG. 13, the correlator 70 shown in FIG. 26 further includes a first inversion unit 320 a and a second inversion unit 320 b, which are generically referred to as inversion units 320. The inversion units 320 invert the signs of the results of addition by a third adder 312 c and a fourth adder 312 d. In other words, the correlator 70 calculate the correlation values between STSb′ having sign inverted from STSb and inputted signals. The operation other than this is the same as in FIG. 13, so that the description thereof is omitted here. By the operation as described above, multiplications between the values of the in-phase component and quadrature component, which a plurality of delayed signals have, and the values of the in-phase component and quadrature component, which a plurality of stored reference signals have, are shared and a plurality of multiplication results produced by the above-mentioned multiplications are added up in different combinations, so that the amount of processing can be reduced.

According to the seventh embodiment of the present invention, a receiving apparatus can recognize the number of antennas transmitting signals without being notified about the number of antennas transmitting the signals from a transmitting apparatus. Also, the circuit can be reduced in scale because the delay units for delaying inputted signals, the storage units for storing reference signals and the multipliers to be used in correlation processing can be shared in the processing for two STSs. Furthermore, this can be realized even with the patterns of STSs for main antenna different from those of the sixth embodiment.

Eighth Embodiment

An eighth embodiment of the present invention, as with the sixth or the seventh embodiment, relates to a technology for a MIMO system, in which a transmitting apparatus changes the pattern of STS with the number of antennas transmitting data and a receiving apparatus recognizes the number of antennas transmitting data by detecting the changed pattern of STS. In the sixth and seventh embodiments, the description was given of the changing of the number of transmitting antennas transmitting data, which is plural, such as “2” or “3”. In this eighth embodiment, however, the description will cover cases where the number of transmitting antennas transmitting data is “1” to “3”, including cases where the number of transmitting antennas is one. It is to be noted here that when the number of transmitting antennas is one, a wireless LAN conforming to the IEEE802.11a standard is assumed, so that the STS corresponding thereto is Legacy STS. The transmitting apparatus transmits Legacy STS, STSa and STSb from the main antenna according to the number of transmitting antennas transmitting data. As previously indicated, Legacy STS uses 12 subcarriers, and STSa and STSb use 6 subcarriers each.

On the other hand, the receiving apparatus, which has correlators corresponding respectively to Legacy STS, STSa and STSb, carries out correlation processings for the signals received by them and outputs their respective correlation values. Furthermore, the receiving apparatus compares the correlation values with one another and specifies the number of the transmitting antennas in response to the STS having the largest correlation value. The receiving apparatus according to the eighth embodiment does not store the values corresponding to the Legacy STS of 12 subcarriers by a correlator corresponding to Legacy STS, but selects 6 subcarriers used by STSa or STSb from the Legacy STS of 12 subcarriers and stores the values corresponding to the thus selected subcarriers.

FIG. 27 illustrates a structure of a correlator 70 according to the eighth embodiment. The correlator 70 includes a correlator for Legacy STS 330, a correlator for STSa 332, a correlator for STSb 334 and a selector 336. Also, correlators corresponding to sub-antennas may be provided, but the description thereof is omitted herein.

The correlator for STSa 332, which stores in advance a plurality of signals where STSa has been converted to signals in the time domain, calculates the correlation value of a stored signal and a received signal (hereinafter referred to as “correlation value for 2 antennas”). The correlator for STSb 334, which stores in advance a plurality of signals where STSb has been converted to signals in the time domain, calculates the correlation value of a stored signal and a received signal (hereinafter referred to as “correlation value for 3 antennas”). Here, the correlator for STSa 332 and the correlator for STSb 334 have been described as having separate structures but they may be structured as a single correlator 70 as in the sixth embodiment.

The correlator for Legacy STS 330 stores in advance the signals, converted into the time domain, of subcarriers used for STSa or STSb only out of the above-described Legacy STS. The correlator for. Legacy STS 330 calculates a correlation value between a stored signal and a received signal (hereinafter referred to as “correlation value for 1 antenna”).

The selector 336 compares a-correlation value for 2 antennas, a correlation value for 3 antennas and a correlation value for 1 antenna and selects a largest correlation value therefrom. An estimation unit, which is not shown here, determines the number of transmitting antennas 14 transmitting data corresponding to the selected correlation value.

According to the eighth embodiment of the present invention, when there is a plurality of antennas transmitting signals, the correlation values are calculated from signals corresponding only to the subcarriers to be used by STSs corresponding to the main antenna. Hence, the influence from the other subcarriers can be eliminated and the accuracy of correlation values for comparison can be raised. Moreover, this improved accuracy of correlation values for comparison raises the accuracy in the estimation of the number of transmitting antennas transmitting signals. Furthermore, a correlator as described above can also be used for timing detection and the like.

Ninth Embodiment

A ninth embodiment of the present invention, as with the first embodiment, relates to preamble signals to be used in a MIMO system. More specifically, this ninth embodiment concerns such placement of preamble signals as can raise the accuracy of gain control by AGC even in a frequency selective fading environment. When a MIMO system comes under the influence of a frequency selective fading, there will occur a plurality of both the large and the small attenuation part of received signals in the frequency band of the signals. For instance, the large and the small attenuation part of signals occur alternately at certain intervals over a range of low to high frequencies. In terms of multicarriers in a MIMO system, a larger attenuation of signals for a predetermined number of subcarriers and then a smaller attenuation of signals for a predetermined number of subcarriers occur, and this is repeated randomly over a range of low-frequency subcarriers to high-frequency subcarriers.

In a MIMO system, as mentioned earlier, it is preferable that the cross-correlations between STSs transmitted respectively from a plurality of antennas are small. However, even when the cross-correlations are small, there may be cases where the signal intensity becomes high for all the subcarriers corresponding to received STSs if the STSs transmitted from their respective antennas use nearly continuous subcarriers, that is, if there is only small frequency difference between the maximum frequency and the minimum frequency of the subcarriers used by one STS. On the contrary, there may also be cases where the signal intensity becomes low. And if a gain setting is done at an AGC unit under any of these circumstances, there is a concern that the value of gain may turn out inappropriate, thus lowering the quality of received signals, when data using a larger number of subcarriers than the STS is received. The reason is that a large number of subcarriers may involve cases where the signal intensity rises or drops.

The STSs corresponding respectively to a plurality of antennas according to the present embodiment use a predetermined number of discretely selected subcarriers. For example, they use subcarriers of every eight subcarrier numbers. This arrangement ensures that even when the number of subcarriers of an STS is smaller than the number of subcarriers of data, the STSs, which use the entire range of the signal band, are subjected to the influence of frequency selective fading not locally but entirely. Also, in consideration of cross-correlations, the STSs corresponding respectively to a plurality of antennas use mutually different subcarriers. Moreover, if the frequency difference between the maximum frequency and the minimum frequency of the subcarriers used by an STS is to be defined as the band width, it is so arranged that the band width is the same for all the plurality of STSs.

If the subcarriers used by a plurality of STSs are placed as described above, then a proper gain can be derived in a frequency selective fading environment even when the number of subcarriers used by an STS is smaller than the number of subcarriers used by data. It is to be noted that the above description of the placement of subcarriers partially overlaps with the description of the first embodiment and others.

A transmitting apparatus 10 and a receiving apparatus 12 according to the ninth embodiment are the same as the transmitting apparatus 10 of FIG. 4 and the receiving apparatus 12 of FIG. 5 according to the first embodiment, so that the description thereof is omitted here.

FIGS. 28A through 28D illustrate, in outline, known signals placed at the subcarriers according to the ninth embodiment. As with FIG. 1, FIGS. 28A through 28D show spectra of signals, the horizontal axis representing the subcarrier numbers and the vertical axis representing the signal strength. Also, the solid line represents the signals of subcarriers during transmission whereas the dotted line represents the transfer function of a channel. As illustrated, the transfer function of a channel is under the influence of frequency selective fading and therefore has parts with high signal strength as well as low signal strength. Here, the parts of the transfer function with high signal strength correspond to parts of the channel subject to smaller attenuation of signals, and the parts of the transfer function with low signal strength correspond to parts of the channel subject to larger attenuation of signals. Different from FIG. 1, the number of subcarriers is assumed here to be 20 to simplify the explanation. Also, the transmitting apparatus 10 is assumed to have two transmitting antennas 14, namely, a first transmitting antenna 14 a and a second transmitting antenna 14 b, and the STSs transmitted from the respective transmitting antennas are assumed to use four subcarriers each.

FIG. 28A shows the placement of subcarriers of STS transmitted from the first transmitting antenna 14 a, with which the placement of subcarriers according to the present embodiment is to be compared. Here, subcarriers of subcarrier numbers “1 to 4” are used. As is shown in FIG. 28A, the attenuation of signals in the channel for the STSs transmitted by the transmitting apparatus 10 is small, so that the signal strength of STSs received by the receiving apparatus 12 is relatively high.

FIG. 28B shows the placement of subcarriers of STS transmitted from the second transmitting antenna 14 b under the same conditions as in FIG. 28A. Here, subcarriers of subcarrier numbers “17 to 20” are used. Here, too, the receiving apparatus 12 receives STSs whose signal strength is relatively high as with the case of FIG. 28A. As a result, if the STSs are placed as shown in FIG. 28A and FIG. 28B, then the receiving apparatus 12 will set the gain of AGC 66 based on the STSs with relatively high signal strength, thus reducing the value of gain.

In the frequency domain corresponding to the subcarrier numbers “4 to 17”, however, the attenuation of signals in the channel is large as is evident in the FIGS. 28A through 28D. Consequently, the signal strength of data using subcarriers in this frequency domain is lower than that of the STSs. In other words, the value of gain set based on the STS becomes smaller than the value of gain appropriate for the transfer function, with the result that errors may sometimes occur in the received signals.

FIG. 28C shows the placement of subcarriers of STS transmitted from the first transmitting antenna 14 a according to the present embodiment. As shown in FIG. 28C, the STS uses a predetermined number of subcarriers discretely selected from a plurality of subcarriers. That is, four subcarriers are used, which have been selected at intervals of five from 20 subcarriers. The subcarrier numbers corresponding thereto are “5”, “10”, “15” and “20”. And the band width is a band corresponding to the number of subcarriers of 15.

FIG. 28D shows the placement of subcarriers of STS transmitted from the second transmitting antenna 14 b under the same conditions as in FIG. 28C. In FIG. 28D, as with the case in FIG. 28C, four subcarriers are used, which have been selected at intervals of five from 20 subcarriers. The subcarrier numbers corresponding thereto, however, are “3”, “8”, “13” and “18”. That is, mutually different subcarriers are used by the STS transmitted by the first transmitting antenna 14 a and the STS transmitted by the second transmitting antenna 14 b. This is done to make the cross-correlation between these STSs smaller. And the band width is equal to the band width for the STS transmitted by the first transmitting antenna 14 a.

In FIG. 28C, the signal strength for the subcarrier numbers “5” and “20” is high while the signal strength for the subcarrier numbers “10” and “15” is low. In FIG. 28D, the signal strength for the subcarrier numbers “3” and “18” is high while the signal strength for the subcarrier numbers “8” and “13” is low. The placement of subcarriers like this involves subcarriers whose signal strength becomes high or low, which reflects qualitatively the transfer function in a frequency selective fading environment. For this reason, the placement of subcarriers as in FIGS. 28C and 28D brings the value of gain closer to the value of gain appropriate for the transfer function. Hence, errors caused in the received signals will be reduced.

Based on specific parameters of an MINO system, the STSs may, for instance, be placed at the subcarriers as follows. The STSs transmitted from the first transmitting antenna 14 a use the subcarriers of subcarrier numbers “−24”, “−16”, “−8”, “4”, “12” and “20” whereas the STSs transmitted from the second transmitting antenna 14 b use the subcarriers of subcarrier numbers “−20”, “−12”, “−4”, “8”, “16” and “24”. Separately from this, the placement may also be such as is expressed by Equation (10) of the first embodiment. It is to be noted that these STSs are stored in preamble adding units 32 as described earlier.

According to the ninth embodiment of the present invention, it is possible to provide preamble signals that improve the accuracy of gain estimation even in a frequency selective fading environment. The ninth embodiment can also make the cross-correlation between preamble signals smaller for a plurality of antennas. Furthermore, since the number of subcarriers used for preambles is small, the cycle of signals in the time domain can be made shorter. Thereby, the gain estimation therefor can be made faster. Moreover, the improved gain estimation can raise the quality of received signals.

Tenth Embodiment

A tenth embodiment according to the present invention relates to a transmitting apparatus in a MIMO system. The transmitting apparatuses according to the above first to ninth embodiments heretofore transmit independent signals from a plurality of respective transmitting antennas. And a plurality of STSs are respectively used to correspond to such the signals. For example, if the signals are to be transmitted from four antennas, respectively, then four kinds of signals will be transmitted. Also, four kinds of STSs are used to correspond respectively to the four kinds of signals. Hereinafter, each of the four kinds of signals will be called series signal, and so far correspond to four series kinds of signals. As a result, in the above first to ninth embodiments, the number of series signals is identical to the number of antennas.

In the tenth embodiment, the number of series signals and the number of transmitting antennas differ from each other. A case is herein considered where the number of series signals is smaller than the number of transmitting antenna. For example, let us assume herein that the number of series signals is “2” whereas the number of transmitting antennas is “4”. In the tenth embodiment, a plurality of series signals are multiplied by a steering matrix, so that the plurality of series signals are dispersed in a plurality of transmitting antennas, respectively. A transmitting apparatus then transmits the dispersed signals from the plurality of antennas, respectively.

FIG. 29 illustrates a structure of a transmitting apparatus according to a tenth embodiment of the present invention. A transmitting apparatus includes an error correcting unit 28, an interleave unit 30, a first modulation unit 110 a and a second modulation unit 110 b, which are generically referred to as modulation units 110, a first preamble adding unit 112 a and a second preamble adding unit 112 b, which are generically referred to as preamble adding units 112, a spatial spreading unit 114, a first radio unit 24 a, a second radio unit 24 b, a third radio unit 24 c and a fourth radio unit 24 d, which are generically referred to as radio units 24, and a first transmitting antenna 14 a, a second transmitting antenna 14 b, a third transmitting antenna 14 c and a fourth transmitting antenna, which are generically referred to as transmitting antennas 14.

Similar to the error correcting unit 28 and the interleave unit 30 shown in FIG. 4, the error correcting unit 28 and the interleave unit 30 according to the tenth embodiment carry out convolutional coding and interleaving, respectively. In FIG. 4, the error correcting unit 28 and the interleave unit 30 perform a processing on the data separated by the data separating unit 20. In contrast thereto, a structure according to the tenth embodiment as shown in FIG. 29 is such that a processing is performed on signals before they are separated. It is to be noted, however, the structure employed in FIG. 4 may be that as shown in FIG. 29. Now, the interleave unit 30 shown in FIG. 29 separates data into a plurality of series and then outputs them. Here, the data are separated into two series. Data of the two series are now independent data from each other.

The modulation units 110 modulate data of two series, respectively. The preamble adding units 112 each add a preamble to the modulated data. Hence, the preamble adding units 112 store respectively a plurality of STSs, corresponding respectively to a plurality of series data, to be transmitted in a predetermined period. Here, STSs stored in the preamble adding units 112 are the same as those described in the above embodiments. That is, an STS corresponding to one of a plurality of series data uses carriers which differ at least partially from carriers used for an STS corresponding to another STS among the plurality of series data. Herein, a unit in which data and STS and the like are combined together will be called a “signal”. It is to be noted that either of data and STS is called a signal and they are used as such without distinction therebetween. Two series data are finally outputted in parallel from the two preamble adding units 112. The burst format of signals outputted from the preamble adding units 112 may be defined as shown in FIGS. 8A to 8C.

The spatial spreading unit 114 inputs the two series signals and multiplies them by a steering matrix so as to generate signals corresponding to the number of transmitting antennas 14. That is, by this multiplication of series signals by the steering matrix the spatial spreading unit 114 generate signals whose count is greater than the number of a plurality of series signals and which correspond to the number of transmitting antennas. The spatial spreading unit 114 extends the degree of an inputted series signal up to “4”. The number of inputted series signal is “2” and is represented here by “Nin”. Hence, the inputted series signal is expressed by a vector “Nin×1”. The number of transmitting antennas 14 is “4” and is represented here by “Nout”. The spatial spreading unit 114 extends the degree of an inputted series signal from Nin to Nout. That is, a vector “Nin×1” is extended to a vector “Nout×1”. In so doing, “0's” are inserted to components from (Nin+1)th row to Nout-th row.

A steering matrix S is expressed by the following Equation 17. S(l)=C(l)W   (17)

The steering matrix is a matrix of “Nout×Nout”. Here, “l” indicates the subcarrier number, and the multiplication by the steering matrix is carried out for each subcarrier. C is expressed by the following Equation 18 and is used for CDD (Cyclic Delay Diversity). C(l)=dia(1,exp(−j2πlδ/Nout), . . . ,exp(−j2πlδ(Nout−1)/Nout))   (18) where δ indicates a shift amount. W is an orthogonal matrix of “Nout×Nout”. One example of the orthogonal matrices is Walsh matrix.

The number of radio units 24 provided is identical to the number of transmitting antennas 14. A structure of each of the radio units 24 here is such that it includes the radio unit 24 shown in FIG. 4 plus the IFFT unit 34, GI unit 36 and quadrature modulation unit 38 shown in FIG. 4. The spatial spreading unit 114 may be provided at a stage subsequent to an IFFT unit 34 (not shown). The radio units 24 output finally signals whose degree has been extended to the number of the transmitting antennas.

A plurality of STSs stored in the preamble adding units 112 may be defined in a manner similar to FIGS. 9A and 9B of the first embodiment. In other words, a plurality of STSs respectively use carries that mutually differ from one another. A plurality of STSs respectively use the a predetermined number of carriers discretely selected from a plurality of carriers. It is defined in a manner such that in the plurality of STSs the number of carriers to be used for the respective STSs are equal to one another. And a plurality of carriers used for a plurality of STSs are predefined among part of a plurality of carriers to be tramsmitted, and the plurality of STSs use at least one carrier selected from the plurality of predefined carriers. The details are similar to those in the first embodiment and are thus omitted here.

The STSs stored in the preamble adding units 112 may be defined in a manner similar to the second embodiment. That is, among the plurality of STSs, the autocorrelation characteristics of STS corresponding to one of a plurality of series signals is defined in such a manner as to be higher than the autocorrelation characteristics of STS corresponding to another series signal among the plurality of series signals. Among the plurality of STSs, the number of carriers to be used for STS corresponding to one of a plurality of series signals is defined in such a manner as to be larger than the number of carriers to be used for STS corresponding to another series signal among the plurality of series signals. The details are similar to those in the second embodiment and are thus omitted here.

The STSs stored in the preamble adding units 112 may also be defined in a manner similar to FIGS. 12A and 12B of the third embodiment. That is, a plurality of STSs are defined in a manner such that a value of in-phase component in a waveform of STS corresponding to one of a plurality of series signals is equal to a value of quadrature component in a waveform of STS corresponding to another of the plurality of series signals and a value of quadrature component in a waveform of STS corresponding to one of plurality of series signals is equal to a value of in-phase component in a waveform of STS corresponding to another of plurality of series signals. The details are similar to those in the third embodiment and are thus omitted here.

The STSs stored in the preamble adding units 112 may also be defined in a manner similar to FIGS. 28C and 28D of the ninth embodiment. That is, a plurality of STSs are respectively defined in such a manner that a difference in frequency between a carrier having the highest frequency and a carrier having the lowest frequency is mutually equal among a predetermined number of discretely selected carriers. The plurality of STSs respectively use mutually different carriers. The details are similar to those in the ninth embodiment and are thus omitted here.

According to the tenth embodiment, a plurality of STSs are required to use mutually different subcarriers from among a plurality of predetermined subcarriers, so that the cross-correlation between the plurality of STSs can be made smaller. Also, because of this smallness of cross-correlation among a plurality of STSs, the detection accuracy of a plurality of STSs by a receiving apparatus can be improved. Moreover, the smallness of cross-correlation among a plurality of STSs can raise the accuracy with which the AGC is set by a receiving apparatus. Moreover, whena plurality of series signals are transmitted from a plurality off transmitting antennas, the number of the plurality of transmitting antennas is larger than the number of the plurality of series signals, so that the strength of signals transmitted form one transmitting antenna can be reduced. Moreover, since the strength of signals transmitted from one transmitting antenna is reduced, so that the distortion, caused by a power amplifier, in the signals to be transmitted can be made smaller. Moreover, an open-loop transmit diversity scheme can be carried out.

The present invention has been described based on the embodiments which are only exemplary. It is understood by those skilled in the art that there exist other various modifications to the combination of each component and process described above and that such modifications are encompassed by the scope of the present invention.

In the first to the tenth embodiment, the signals to be considered as reference signals are STSs defined in IEEE802.11a standard. However, they are not limited thereto and, for example, other signals may serve the purpose. That is, they may be known signal that are placed and transmitted in a plurality of subcarriers and transmitted.

In the ninth embodiment, the mutually different subcarriers are used as a plurality of STSs stored in a transmitting apparatus 10. However, they are not limited thereto and, for example, subcarriers that partially or entirely overlapped may be used. In such a case, it is preferable that the pattern of STS with which cross-correlation among a plurality of STSs becomes low be used. This is because in such the case there is a possibility where the cross-correlation among the plurality of STSs becomes high. According to this modified example, the advantageous effect as in the second embodiment is obtained. That is, it suffices if the band width is wide to some extent.

In the ninth embodiment, the plurality of STSs stored in the transmitting apparatus 10 are so defined that the band widths are equal. However, they are not limited thereto and, for example, they may be set to band widths whose values differ for one another. According to this modified example, the present invention can be applied to a case when the number of transmitting antennas is larger. That is, it suffices if the band width is wide to some extent.

In the seventh embodiment, the transmitting apparatus 10 changes the pattern of STS according to the number of transmitting antennas to be used and the receiving apparatus 12 recognizes the number of transmitting antennas in use by detecting the changed pattern of STS. The patterns of STSs transmitted from a main antenna are given a predetermined relationship according to the number of transmitting antennas 14 transmitting data. In particular, the predetermined relationship is defined in a relationship such that the absolute values of the respective components are exchanged and the sings are inverted. It is, however, not limited thereto and, for instance, a predetermined relationship may be defined respectively for a plurality of transmitting antennas 14. That is, the predetermined relationship that a plurality of STSs stored in the preamble adding unit 32 have is such that the absolute value of in-phase component in a waveform of STS corresponding to one of the plurality of transmitting antennas 14 is equal to the absolute value of quadrature component in a waveform of STS corresponding to another of the plurality of transmitting antennas 14 and the sign thereof is inverted.

Moreover, it may be defined such that the absolute value of quadrature component in a waveform of STS corresponding to one of the plurality of transmitting antennas 14 is equal to the absolute value of in-phase component in a waveform of STS corresponding to another of the plurality of transmitting antennas 14 and the sign thereof is inverted. If this is applied to the third embodiment, the waveform of STS corresponding to one of the plurality of transmitting antennas 14 corresponds to the waveform of STS corresponding to the second transmitting antenna 14 b, and the waveform of STS corresponding to another of the plurality of transmitting antennas 14 corresponds to the waveform of STS corresponding to the third transmitting antenna 14 c.

Moreover, the above modified examples may be applied to the tenth embodiment. That is, STSs stored in the preamble adding unit 112 may be defined in a manner such that the absolute value of in-phase component in a waveform of STS corresponding to one of a plurality of series signals is equal to the absolute value of quadrature component in a waveform of STS corresponding to another of the plurality of series signals and the sign thereof is inverted. Moreover, they may be defined in a manner such that the absolute value of quadrature component in a waveform of STS corresponding to one of the plurality of series signals is equal to the absolute value of in-phase component in a waveform of STS corresponding to another of the plurality of series signals and the sign thereof is inverted. According to this modified example, the correlation among STSs corresponding to a plurality of transmitting antennas 14 can be made small.

Although the present invention has been described by way of exemplary embodiments, it should be understood that many changes and substitutions may further be made by those skilled in the art without departing from the scope of the present invention which is defined by the appended claims. 

1. A correlator, comprising: an input unit which sequentially inputs signals in which a plurality of series are combined wherein the plurality of series are defined beforehand in a form such that a plurality of reference signals among the series are respectively changed for the series in which the plurality of reference signals are arranged on time axis; a delay unit which continuously delays the inputted signals; a plurality of storage units which store respectively a plurality of reference signals corresponding one of the plurality of series; and a correlation processing unit which performs correlation processing based on values of the plurality of delayed signals and values of the plurality of stored reference signals and which outputs respectively a plurality of values of correlation between the inputted signals and the plurality of series, wherein a plurality of series contained in the signals inputted to said input unit have a predetermined relationship, and wherein said correlation processing unit performs the correlation processing by combination of multiplication and addition according to the predetermined relationship.
 2. A correlator according to claim 1, wherein the signals inputted to said input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have is such that a value of in-phase component of a reference signal contained in one of the plurality of series is equal to a value of quadrature component of a reference signal contained in the other thereof whereas a value of quadarature component of the reference signal contained in one of the plurality of series is equal to a value of in-phase component of the reference signal contained in the other thereof, and wherein said correlation processing unit puts to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sums up in different combinations a plurality of multiplication results generated by the multiplication.
 3. A correlator according to claim 2, wherein if a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value-of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then said correlation processing unit performs the correlation processing in a manner such that a correlation processing, the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and a difference between the second value and the first value are calculated for the other of the two series.
 4. A correlator according to claim 2, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 5. A correlator according to claim 3, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of a reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of a reference signal contained in a series corresponding to the another of the number of antennas, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 6. A correlator according to claim 1, wherein the signals inputted to said input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have is such that an absolute value of in-phase component of a reference signal contained in one of the plurality of series is equal to an absolute value of quadrature component of a reference signal contained in the other thereof and the sign thereof is inverted whereas an absolute value of quadarature component of the reference signal contained in one of the plurality of series is equal to an absolute value of in-phase component of the reference signal contained in the other thereof and the sign thereof is inverted, and wherein said correlation processing unit puts to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sums up in different combinations a plurality of multiplication results generated by the multiplication.
 7. A correlator according to claim 6, wherein if a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then said correlation processing unit performs the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and then inverting the sign of resulting summation and a difference between the second value and the first value and then inverting the sign of resulting difference are calculated for the other of the two series.
 8. A correlator according to claim 6, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to an absolute value of in-phase component of the reference signal contained in a series corresponding to-the another of the number of antennas and the sign thereof is inverted, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 9. A correlator according to claim 7, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to an absolute value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas and the sign thereof is inverted, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 10. A correlator according to claim 1, wherein the plurality of series contained in the signals inputted to said input unit are defined in a manner such that each value thereof varies in a predetermined cycle, and the predetermined relationship is defined in a manner such that if one of the plurality of series is assigned as a reference, then a cycle of the series assigned as the reference is an integral multiple of a cycle of series other than the series assigned as the reference, wherein said plurality of storage units store respectively a plurality of reference signals corresponding to the series assigned as the reference, and wherein said correlation processing unit selects part of the plurality of stored reference signals for the series other than the series assigned as the reference, based on a difference between the cycle of series assigned as the reference and the cycle of series other than the series assigned as the reference, and carries out multiplication between the thus partially selected reference signal and the plurality of delayed input signals.
 11. A correlator according to claim 1, wherein the input signals inputted to said input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have is such that only one of in-phase component and quadrature component of the reference signal has a predetermined value, wherein said storage unit respectively stores information by which to invert the sign of the delayed signals in such a form as to correspond to the plurality of reference signals, and wherein as the correlation processing said correlation processing unit adds up a signal which has been inverted based on the information and the delayed signal.
 12. A receiving apparatus, comprising: an input unit which sequentially inputs signals in which a plurality of series are combined wherein the plurality of series are defined beforehand in a form such that a plurality of reference signals among the series are respectively changed for the series in which the plurality of reference signals are arranged on time axis; a delay unit which continuously delays the inputted signals; a plurality of storage units which store respectively a plurality of reference signals corresponding one of the plurality of series; a correlation processing unit which performs correlation processing based on values of the plurality of delayed signals and values of the plurality of stored reference signals and which outputs respectively a plurality of values of correlation between the inputted signals and the plurality of series; and a control unit which detects timing of the inputted signals based on the plurality of values of correlation, wherein a plurality of series contained in the signals inputted to said input unit have a predetermined relationship, and wherein said correlation processing unit performs the correlation processing by combination of multiplication and addition according to the predetermined relationship.
 13. A receiving apparatus according to claim 12, wherein the signals inputted to said input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have is such that a value of in-phase component of a reference signal contained in one of the plurality of series is equal to a value of quadrature component of a reference signal contained in the other thereof whereas a value of quadarature component of the reference signal contained in one of the plurality of series is equal to a value of in-phase component of the reference signal contained in the other thereof, and wherein said correlation processing unit puts to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sums up in different combinations a plurality of multiplication results generated by the multiplication.
 14. A receiving apparatus according to claim 13, wherein if a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then said correlation processing unit performs the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and a difference between the second value and the first value are calculated for the other of the two series.
 15. A receiving apparatus according to claim 13, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 16. A receiving apparatus according to claim 14, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that a value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to a value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and a value of quadrature component of a reference signal contained in a series corresponding to the predetermined antenna is equal to a value of in-phase component of a reference signal contained in a series corresponding to the another of the number of antennas, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 17. A receiving apparatus according to claim 12, wherein the signals inputted to said input unit and the plurality of reference signals have in-phase components and quadrature components, and the predetermined relationship that the plurality of series have is such that an absolute value of in-phase component of a reference signal contained in one of the plurality of series is equal to an absolute value of quadrature component of a reference signal contained in the other thereof and the sign thereof is inverted whereas an absolute value of quadarature component of the reference signal contained in one of the plurality of series is equal to an absolute value of in-phase component of the reference signal contained in the other thereof and the sign thereof is inverted, and wherein said correlation processing unit puts to a common use the multiplication between values of in-phase components and values of quadrature components that the plurality of delayed signals have and values of in-phase components and values of quadrature components that the plurality of stored reference signals have, and sums up in different combinations a plurality of multiplication results generated by the multiplication.
 18. A receiving apparatus according to claim 17, wherein if a result of multiplication of a value of in-phase component that the signal has and a value of in-phase component that the reference signal has is denoted as a first value, a result of multiplication of a value of quadrature component that the signal has and a value of quadrature component that the reference signal has is a second value, a result of multiplication of a value of in-phase component that the signal has and a value of quadrature component that the reference signal has is a third value and a result of multiplication of a value of quadrature component that the signal has and a value of in-phase component that the reference signal has is a fourth value, then said correlation processing unit performs the correlation processing in a manner such that the sum of the first value and the second value and a difference between the fourth value and the third value are calculated for one of two series whereas the sum of the third value and the fourth value and then inverting the sign of resulting summation and a difference between the second value and the first value and then inverting the sign of resulting difference are calculated for the other of the two series.
 19. A receiving apparatus according to claim 17, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to,an absolute value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas and the sign thereof is inverted, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals.
 20. A receiving apparatus according to claim 18, further comprising a determination unit which determines the number of antennas that are to transmit signals, based on the plurality of values of correlation, wherein signals to be inputted to said input unit are transmitted from a plurality of transmitting antennas whose number is being varied, and if one of the plurality of antennas is called a main antenna and the remaining antennas are called sub-antennas, then there is a relationship where, according to the number of antennas that transmit signals, a series that is to be transmitted from the main antenna is such that an absolute value of in-phase component of a reference signal contained in a series corresponding to a predetermined number of antennas that transmit signals is equal to an absolute value of quadrature component of a reference signal contained in a series corresponding to another number of antennas and the sign thereof is inverted whereas an absolute value of quadrature component of the reference signal contained in a series corresponding to the predetermined antenna is equal to an absolute value of in-phase component of the reference signal contained in a series corresponding to the another of the number of antennas and the sign thereof is inverted, and wherein said storage unit stores, among series to be transmitted from the main antenna, a plurality of reference signals contained in a series corresponding to a predetermined number of antennas that transmit signals. 